[Function add]

1. Add leetcode solutions using C++.

Author: Seanforfun

Diff for: leetcode/127. Word Ladder.md

@@ -266,6 +266,44 @@ class Solution {
     }
     ```
 
+### C++ Version
+* Method 1: bi-directional bfs
+	```objectivec
+	class Solution {
+	public:
+		int ladderLength(string beginWord, string endWord, vector<string>& wordList) {
+			unordered_set<string> set(wordList.begin(), wordList.end());
+			if(set.find(endWord) == set.end()) return 0;
+			set.erase(endWord);
+			unordered_set<string> first{beginWord}, second{endWord};
+			int len = endWord.length();
+			int step = 0;
+			while(!first.empty() && !second.empty()){
+				++step;
+				if(first.size() > second.size())
+					std::swap(first, second);
+				unordered_set<string> next;
+				for(string cur: first){
+					for(int i = 0; i < len; ++i){
+						char origin = cur[i];
+						for(char c = 'a'; c <= 'z'; ++c){
+							cur[i] = c;
+							if(second.find(cur) != second.end())    return step + 1;
+							if(set.find(cur) != set.end()){
+								next.emplace(cur);
+								set.erase(cur);
+							}
+							cur[i] = origin;
+						}
+					}
+				}
+				std::swap(next, first);
+			}
+			return 0;
+		}
+	};
+	```
+
 ### Conclusion
 * How to choose dfs and bfs
     1. bfs is designed for shortest path question.