add substring with concatenation

tg123 · tg123 · commit 749eb78a13c9 · 2014-08-23T00:06:08.000+08:00
diff --git a/substring-with-concatenation-of-all-words/README.md b/substring-with-concatenation-of-all-words/README.md
@@ -1,4 +1,38 @@
+## [Space-time tradeoff](http://en.wikipedia.org/wiki/Space%E2%80%93time_tradeoff)
 
-## TODO 
-  * write down thinking
+A brute force solution is easy. Yet, obviously, time limited. 
 
+### Check concatenation using count sum
+
+cut a string `S` into N substring, lets say `s1 * 1, s2 * 2, s3 * 1`, 
+that is, `S` could be `s2s1s2s3` or `s3s1s2s2` or some string else.
+
+convert `L` to count sum, which is a map of `string to count`
+
+```
+M = empty Map
+
+for s in L
+  M[s] = M[s] + 1
+  
+
+```
+
+
+check concatenation S by minus count
+
+```
+cut S into substrings by some length
+
+_M = clone of M
+
+for s in substrings
+  
+  if _M[s] == 0
+    Not a concatenation
+ 
+  _M[s] = _M[s] - 1
+  
+
+
+```
