这里的题目难度比较小， 大多是模拟题，或者是很容易看出解法的题目，另外简单题目一般使用暴力法都是可以解决的。 这个时候只有看一下数据范围，思考下你的算法复杂度就行了。

当然也不排除很多 hard 题目也可以暴力模拟，大家平时多注意数据范围即可。

以下是我列举的经典题目（带 91 字样的表示出自 **91 天学算法**活动）：

- [面试题 17.12. BiNode](./problems/binode-lcci.md)

- [0001. 两数之和](./problems/1.two-sum.md)

- [0020. 有效的括号](./problems/20.valid-parentheses.md)

- [0021. 合并两个有序链表](./problems/21.merge-two-sorted-lists.md)

- [0026. 删除排序数组中的重复项](./problems/26.remove-duplicates-from-sorted-array.md)

- [0053. 最大子序和](./problems/53.maximum-sum-subarray-cn.md)

- [0066. 加一](./problems/66.plus-one.md) 91

- [0088. 合并两个有序数组](./problems/88.merge-sorted-array.md)

- [0101. 对称二叉树](./problems/101.symmetric-tree.md)

- [0104. 二叉树的最大深度](./problems/104.maximum-depth-of-binary-tree.md)

- [0108. 将有序数组转换为二叉搜索树](./problems/108.convert-sorted-array-to-binary-search-tree.md)

- [0121. 买卖股票的最佳时机](./problems/121.best-time-to-buy-and-sell-stock.md)

- [0122. 买卖股票的最佳时机 II](./problems/122.best-time-to-buy-and-sell-stock-ii.md)

- [0125. 验证回文串](./problems/125.valid-palindrome.md)

- [0136. 只出现一次的数字](./problems/136.single-number.md)

- [0155. 最小栈](./problems/155.min-stack.md)

- [0160. 相交链表](./problems/160.Intersection-of-Two-Linked-Lists.md) 91

- [0167. 两数之和 II 输入有序数组](./problems/167.two-sum-ii-input-array-is-sorted.md)

- [0169. 多数元素](./problems/169.majority-element.md)

- [0172. 阶乘后的零](./problems/172.factorial-trailing-zeroes.md)

- [0190. 颠倒二进制位](./problems/190.reverse-bits.md)

- [0191. 位 1 的个数](./problems/191.number-of-1-bits.md)

- [0198. 打家劫舍](./problems/198.house-robber.md)

- [0203. 移除链表元素](./problems/203.remove-linked-list-elements.md)

- [0206. 反转链表](./problems/206.reverse-linked-list.md)

- [0219. 存在重复元素 II](./problems/219.contains-duplicate-ii.md)

- [0226. 翻转二叉树](./problems/226.invert-binary-tree.md)

- [0232. 用栈实现队列](./problems/232.implement-queue-using-stacks.md) 91

- [0263. 丑数](./problems/263.ugly-number.md)

- [0283. 移动零](./problems/283.move-zeroes.md)

- [0342. 4 的幂](./problems/342.power-of-four.md)

- [0349. 两个数组的交集](./problems/349.intersection-of-two-arrays.md)

- [0371. 两整数之和](./problems/371.sum-of-two-integers.md)

- [401. 二进制手表](./problems/401.binary-watch.md)

- [0437. 路径总和 III](./problems/437.path-sum-iii.md)

- [0455. 分发饼干](./problems/455.AssignCookies.md)

- [0575. 分糖果](./problems/575.distribute-candies.md)

- [821. 字符的最短距离](./problems/821.shortest-distance-to-a-character.md) 91

- [0874. 模拟行走机器人](./problems/874.walking-robot-simulation.md)

- [1260. 二维网格迁移](./problems/1260.shift-2d-grid.md)

- [1332. 删除回文子序列](./problems/1332.remove-palindromic-subsequences.md)

中等题目是力扣比例最大的部分，因此这部分我的题解也是最多的。 大家不要太过追求难题，先把中等难度题目做熟了再说。

这部分的题目要不需要我们挖掘题目的内含信息， 将其抽象成简单题目。 要么是一些写起来比较麻烦的题目， 一些人编码能力不行就挂了。因此大家一定要自己做， 即使看了题解”会了“，也要自己码一遍。自己不亲自写一遍，里面的细节永远不知道。

以下是我列举的经典题目（带 91 字样的表示出自 **91 天学算法**活动）：

- [面试题 17.09. 第 k 个数](./problems/get-kth-magic-number-lcci.md)

- [面试题 17.23. 最大黑方阵](./problems/max-black-square-lcci.md)🆕

- [0002. 两数相加](./problems/2.add-two-numbers.md)

- [0003. 无重复字符的最长子串](./problems/3.longest-substring-without-repeating-characters.md)

- [0005. 最长回文子串](./problems/5.longest-palindromic-substring.md)

- [0011. 盛最多水的容器](./problems/11.container-with-most-water.md)

- [0015. 三数之和](./problems/15.3sum.md)

- [0017. 电话号码的字母组合](./problems/17.Letter-Combinations-of-a-Phone-Number.md)

- [0019. 删除链表的倒数第 N 个节点](./problems/19.removeNthNodeFromEndofList.md)

- [0022. 括号生成](./problems/22.generate-parentheses.md)

- [0024. 两两交换链表中的节点](./problems/24.swapNodesInPairs.md)

- [0029. 两数相除](./problems/29.divide-two-integers.md)

- [0031. 下一个排列](./problems/31.next-permutation.md)

- [0033. 搜索旋转排序数组](./problems/33.search-in-rotated-sorted-array.md)

- [0039. 组合总和](./problems/39.combination-sum.md)

- [0040. 组合总和 II](./problems/40.combination-sum-ii.md)

- [0046. 全排列](./problems/46.permutations.md)

- [0047. 全排列 II](./problems/47.permutations-ii.md)

- [0048. 旋转图像](./problems/48.rotate-image.md)

- [0049. 字母异位词分组](./problems/49.group-anagrams.md)

- [0050. Pow(x, n)](./problems/50.pow-x-n.md)

- [0055. 跳跃游戏](./problems/55.jump-game.md)

- [0056. 合并区间](./problems/56.merge-intervals.md)

- [0060. 第 k 个排列](./problems/60.permutation-sequence.md)

- [0061. 旋转链表](./problems/61.Rotate-List.md) 91

- [0062. 不同路径](./problems/62.unique-paths.md)

- [0073. 矩阵置零](./problems/73.set-matrix-zeroes.md)

- [0075. 颜色分类](./problems/75.sort-colors.md)

- [0078. 子集](./problems/78.subsets.md)

- [0079. 单词搜索](./problems/79.word-search.md)

- [0080. 删除排序数组中的重复项 II](./problems/80.remove-duplicates-from-sorted-array-ii.md)

- [0086. 分隔链表](./problems/86.partition-list.md)

- [0090. 子集 II](./problems/90.subsets-ii.md)

- [0091. 解码方法](./problems/91.decode-ways.md)

- [0092. 反转链表 II](./problems/92.reverse-linked-list-ii.md)

- [0094. 二叉树的中序遍历](./problems/94.binary-tree-inorder-traversal.md)

- [0095. 不同的二叉搜索树 II](./problems/95.unique-binary-search-trees-ii.md)

- [0096. 不同的二叉搜索树](./problems/96.unique-binary-search-trees.md)

- [0098. 验证二叉搜索树](./problems/98.validate-binary-search-tree.md)

- [0102. 二叉树的层序遍历](./problems/102.binary-tree-level-order-traversal.md)

- [0103. 二叉树的锯齿形层次遍历](./problems/103.binary-tree-zigzag-level-order-traversal.md)

- [0113. 路径总和 II](./problems/113.path-sum-ii.md)

- [0129. 求根到叶子节点数字之和](./problems/129.sum-root-to-leaf-numbers.md)

- [0130. 被围绕的区域](./problems/130.surrounded-regions.md)

- [0131. 分割回文串](./problems/131.palindrome-partitioning.md)

- [0139. 单词拆分](./problems/139.word-break.md)

- [0144. 二叉树的前序遍历](./problems/144.binary-tree-preorder-traversal.md)

- [0147. 对链表进行插入排序](./problems/147.insertion-sort-list.md) 🆕

- [0150. 逆波兰表达式求值](./problems/150.evaluate-reverse-polish-notation.md)

- [0152. 乘积最大子数组](./problems/152.maximum-product-subarray.md)

- [0199. 二叉树的右视图](./problems/199.binary-tree-right-side-view.md)

- [0200. 岛屿数量](./problems/200.number-of-islands.md)

- [0201. 数字范围按位与](./problems/201.bitwise-and-of-numbers-range.md)

- [0208. 实现 Trie (前缀树)](./problems/208.implement-trie-prefix-tree.md)

- [0209. 长度最小的子数组](./problems/209.minimum-size-subarray-sum.md)

- [0211. 添加与搜索单词 \* 数据结构设计](./problems/211.add-and-search-word-data-structure-design.md)

- [0215. 数组中的第 K 个最大元素](./problems/215.kth-largest-element-in-an-array.md)

- [0221. 最大正方形](./problems/221.maximal-square.md)

- [0227. 基本计算器 II](./problems/227.basic-calculator-ii.md)🆕

- [0229. 求众数 II](./problems/229.majority-element-ii.md)

- [0230. 二叉搜索树中第 K 小的元素](./problems/230.kth-smallest-element-in-a-bst.md)

- [0236. 二叉树的最近公共祖先](./problems/236.lowest-common-ancestor-of-a-binary-tree.md)

- [0238. 除自身以外数组的乘积](./problems/238.product-of-array-except-self.md)

- [0240. 搜索二维矩阵 II](./problems/240.search-a-2-d-matrix-ii.md)

- [0279. 完全平方数](./problems/279.perfect-squares.md)

- [0309. 最佳买卖股票时机含冷冻期](./problems/309.best-time-to-buy-and-sell-stock-with-cooldown.md)

- [0322. 零钱兑换](./problems/322.coin-change.md)

- [0328. 奇偶链表](./problems/328.odd-even-linked-list.md)

- [0334. 递增的三元子序列](./problems/334.increasing-triplet-subsequence.md)

- [0337. 打家劫舍 III](./problems/337.house-robber-iii.md)

- [0343. 整数拆分](./problems/343.integer-break.md)

- [0365. 水壶问题](./problems/365.water-and-jug-problem.md)

- [0378. 有序矩阵中第 K 小的元素](./problems/378.kth-smallest-element-in-a-sorted-matrix.md)

- [0380. 常数时间插入、删除和获取随机元素](./problems/380.insert-delete-getrandom-o1.md)

- [0394. 字符串解码](./problems/394.decode-string.md) 91

- [0416. 分割等和子集](./problems/416.partition-equal-subset-sum.md)

- [0445. 两数相加 II](./problems/445.add-two-numbers-ii.md)

- [0454. 四数相加 II](./problems/454.4-sum-ii.md)

- [0464. 我能赢么](./problems/464.can-i-win.md)

- [0494. 目标和](./problems/494.target-sum.md)

- [0516. 最长回文子序列](./problems/516.longest-palindromic-subsequence.md)

- [0513. 找树左下角的值](./problems/513.find-bottom-left-tree-value.md) 91

- [0518. 零钱兑换 II](./problems/518.coin-change-2.md)

- [0547. 朋友圈](./problems/547.friend-circles.md)

- [0560. 和为 K 的子数组](./problems/560.subarray-sum-equals-k.md)

- [0609. 在系统中查找重复文件](./problems/609.find-duplicate-file-in-system.md)

- [0611. 有效三角形的个数](./problems/611.valid-triangle-number.md)

- [0686. 重复叠加字符串匹配](./problems/686.repeated-string-match.md) 🆕

- [0718. 最长重复子数组](./problems/718.maximum-length-of-repeated-subarray.md)

- [0754. 到达终点数字](./problems/754.reach-a-number.md)

- [0785. 判断二分图](./problems/785.is-graph-bipartite.md)

- [0816. 模糊坐标](./problems/816.ambiguous-coordinates.md) 🆕

- [0820. 单词的压缩编码](./problems/820.short-encoding-of-words.md)

- [0875. 爱吃香蕉的珂珂](./problems/875.koko-eating-bananas.md)

- [0877. 石子游戏](./problems/877.stone-game.md)

- [0886. 可能的二分法](./problems/886.possible-bipartition.md)

- [0900. RLE 迭代器](./problems/900.rle-iterator.md)

- [0911. 在线选举](./problems/911.online-election.md) 🆕

- [0912. 排序数组](./problems/912.sort-an-array.md)

- [0935. 骑士拨号器](./problems/935.knight-dialer.md)

- [0978. 最长湍流子数组](./problems/978.longest-turbulent-subarray.md) 🆕

- [0987. 二叉树的垂序遍历](./problems/987.vertical-order-traversal-of-a-binary-tree.md) 91

- [1011. 在 D 天内送达包裹的能力](./problems/1011.capacity-to-ship-packages-within-d-days.md)

- [1014. 最佳观光组合](./problems/1014.best-sightseeing-pair.md)

- [1015. 可被 K 整除的最小整数](./problems/1015.smallest-integer-divisible-by-k.md)

- [1019. 链表中的下一个更大节点](./problems/1019.next-greater-node-in-linked-list.md)

- [1020. 飞地的数量](./problems/1020.number-of-enclaves.md)

- [1023. 驼峰式匹配](./problems/1023.camelcase-matching.md)

- [1031. 两个非重叠子数组的最大和](./problems/1031.maximum-sum-of-two-non-overlapping-subarrays.md)

- [1104. 二叉树寻路](./problems/1104.path-in-zigzag-labelled-binary-tree.md)

- [1131.绝对值表达式的最大值](./problems/1131.maximum-of-absolute-value-expression.md)

- [1186. 删除一次得到子数组最大和](./problems/1186.maximum-subarray-sum-with-one-deletion.md)

- [1218. 最长定差子序列](./problems/1218.longest-arithmetic-subsequence-of-given-difference.md)

- [1227. 飞机座位分配概率](./problems/1227.airplane-seat-assignment-probability.md)

- [1261. 在受污染的二叉树中查找元素](./problems/1261.find-elements-in-a-contaminated-binary-tree.md)

- [1262. 可被三整除的最大和](./problems/1262.greatest-sum-divisible-by-three.md)

- [1297. 子串的最大出现次数](./problems/1297.maximum-number-of-occurrences-of-a-substring.md)

- [1310. 子数组异或查询](./problems/1310.xor-queries-of-a-subarray.md)

- [1334. 阈值距离内邻居最少的城市](./problems/1334.find-the-city-with-the-smallest-number-of-neighbors-at-a-threshold-distance.md)

- [1371.每个元音包含偶数次的最长子字符串](./problems/1371.find-the-longest-substring-containing-vowels-in-even-counts.md)

- [1381. 设计一个支持增量操作的栈](./problems/1381.design-a-stack-with-increment-operation.md) 91

- [1558. 得到目标数组的最少函数调用次数](./problems/1558.minimum-numbers-of-function-calls-to-make-target-array.md) 🆕

- [1574. 删除最短的子数组使剩余数组有序](./problems/1574.shortest-subarray-to-be-removed-to-make-array-sorted.md) 🆕

- [1631. 最小体力消耗路径](./problems/1631.path-with-minimum-effort.md) 🆕

- [1658. 将 x 减到 0 的最小操作数](./problems/1658.minimum-operations-to-reduce-x-to-zero.md) 🆕

困难难度题目从类型上说多是：

- 图

- 设计题

- 游戏场景题目

- 中等题目的 follow up

从解法上来说，多是：

- 图算法

- 动态规划

- 二分法

- DFS & BFS

- 状态压缩

- 剪枝

从逻辑上说， 要么就是非常难想到，要么就是非常难写代码。 这里我总结了几个技巧：

1. 看题目的数据范围， 看能否暴力模拟

2. 暴力枚举所有可能的算法往上套，比如图的题目。

3. 总结和记忆解题模板，减少解题压力

以下是我列举的经典题目（带 91 字样的表示出自 **91 天学算法**活动）：

- [0004. 寻找两个正序数组的中位数](./problems/4.median-of-two-sorted-arrays.md)

- [0023. 合并 K 个升序链表](./problems/23.merge-k-sorted-lists.md)

- [0025. K 个一组翻转链表](./problems/25.reverse-nodes-in-k-groups.md)

- [0030. 串联所有单词的子串](./problems/30.substring-with-concatenation-of-all-words.md)

- [0032. 最长有效括号](./problems/32.longest-valid-parentheses.md)

- [0042. 接雨水](./problems/42.trapping-rain-water.md)

- [0052. N 皇后 II](./problems/52.N-Queens-II.md)

- [0057. 插入区间](problems/57.insert-interval.md) 🆕

- [0084. 柱状图中最大的矩形](./problems/84.largest-rectangle-in-histogram.md)

- [0085. 最大矩形](./problems/85.maximal-rectangle.md)

- [0124. 二叉树中的最大路径和](./problems/124.binary-tree-maximum-path-sum.md)

- [0128. 最长连续序列](./problems/128.longest-consecutive-sequence.md)

- [0140. 单词拆分 II](problems/140.word-break-ii.md) 🆕

- [0145. 二叉树的后序遍历](./problems/145.binary-tree-postorder-traversal.md)

- [0212. 单词搜索 II](./problems/212.word-search-ii.md)

- [0239. 滑动窗口最大值](./problems/239.sliding-window-maximum.md)

- [0295. 数据流的中位数](./problems/295.find-median-from-data-stream.md)

- [0297. 二叉树的序列化与反序列化](./problems/297.serialize-and-deserialize-binary-tree.md) 91

- [0301. 删除无效的括号](./problems/301.remove-invalid-parentheses.md)

- [0312. 戳气球](./problems/312.burst-balloons.md)

- [330. 按要求补齐数组](./problems/330.patching-array.md)

- [0335. 路径交叉](./problems/335.self-crossing.md)

- [0460. LFU 缓存](./problems/460.lfu-cache.md)

- [0472. 连接词](./problems/472.concatenated-words.md)

- [0488. 祖玛游戏](./problems/488.zuma-game.md)

- [0493. 翻转对](./problems/493.reverse-pairs.md)

- [0715. Range 模块](./problems/715.range-module.md) 🆕

- [0768. 最多能完成排序的块 II](./problems/768.max-chunks-to-make-sorted-ii.md) 91

- [0887. 鸡蛋掉落](./problems/887.super-egg-drop.md)

- [0895. 最大频率栈](./problems/895.maximum-frequency-stack.md)

- [0975. 奇偶跳](./problems/975.odd-even-jump.md) 🆕

- [1032. 字符流](./problems/1032.stream-of-characters.md)

- [1168. 水资源分配优化](./problems/1168.optimize-water-distribution-in-a-village.md)

- [1203. 项目管理](./problems/1203.sort-items-by-groups-respecting-dependencies.md) 🆕

- [1255. 得分最高的单词集合](./problems/1255.maximum-score-words-formed-by-letters.md)

- [1345. 跳跃游戏 IV](./problems/1435.jump-game-iv.md)

- [1449. 数位成本和为目标值的最大数字](./problems/1449.form-largest-integer-with-digits-that-add-up-to-target.md) 🆕

- [5640. 与数组中元素的最大异或值](./problems/5640.maximum-xor-with-an-element-from-array.md) 🆕

* [0975. 奇偶跳](../problems/975.odd-even-jump.md) 🆕