src/main/java/com/fishercoder/solutions/_554.java

package com.fishercoder.solutions;

import java.util.HashMap;
import java.util.List;
import java.util.Map;

/**
 * 554. Brick Wall
 *
 * There is a brick wall in front of you. The wall is rectangular and has several rows of bricks.
 * The bricks have the same height but different width.
 * You want to draw a vertical line from the top to the bottom and cross the least bricks.
 * The brick wall is represented by a list of rows.
 * Each row is a list of integers representing the width of each brick in this row from left to right.
 * If your line go through the edge of a brick,
 * then the brick is not considered as crossed.
 * You need to find out how to draw the line to cross the least bricks and return the number of crossed bricks.
 * You cannot draw a line just along one of the two vertical edges of the wall, in which case the line will obviously cross no bricks.

 Example:
 Input:
 [[1,2,2,1],
 [3,1,2],
 [1,3,2],
 [2,4],
 [3,1,2],
 [1,3,1,1]]

 Output: 2
 Explanation:

 Note:
 The width sum of bricks in different rows are the same and won't exceed INT_MAX.
 The number of bricks in each row is in range [1,10,000]. The height of wall is in range [1,10,000]. Total number of bricks of the wall won't exceed 20,000.
 */
public class _554 {
    public static class Solution1 {
        /**
         * credit: https://leetcode.com/articles/brick-wall/
         *
         * we make use of a HashMap
         * map which is used to store entries in the form:
         * (sum,count). Here,
         * sum refers to the cumulative sum of the bricks' widths encountered in the current row, and
         * count refers to the number of times the corresponding sum is obtained. Thus,
         * sum in a way, represents the positions of the bricks's boundaries relative to the leftmost boundary.
         *
         * This is done based on the following observation:
         * We will never obtain the same value of sum twice while traversing over a particular row.
         * Thus, if the sum value is repeated while traversing over the rows, it means some row's brick boundary coincides with some previous row's brick boundary.
         * This fact is accounted for by incrementing the corresponding count value.
         * But, for every row, we consider the sum only upto the second last brick, since the last boundary isn't a valid boundary for the solution.
         */

        public int leastBricks(List<List<Integer>> wall) {
            Map<Integer, Integer> map = new HashMap();
            for (List<Integer> row : wall) {
                int sum = 0;
                for (int i = 0; i < row.size() - 1; i++) {
                    //NOTE: i < row.size()-1
                    sum += row.get(i);
                    if (map.containsKey(sum)) {
                        map.put(sum, map.get(sum) + 1);
                    } else {
                        map.put(sum, 1);
                    }
                }
            }
            int result = wall.size();
            for (int key : map.keySet()) {
                result = Math.min(result, wall.size() - map.get(key));
            }
            return result;
        }
    }
}