## Description

LeetCode Problem 127.

A transformation sequence from word beginWord to word endWord using a dictionary wordList is a sequence of words beginWord -> s_1 -> s_2 -> … -> s_k such that:

• Every adjacent pair of words differs by a single letter.
• Every s_i for 1 <= i <= k is in wordList. Note that beginWord does not need to be in wordList.
• s_k == endWord

Given two words, beginWord and endWord, and a dictionary wordList, return the number of words in the shortest transformation sequence from beginWord to endWord, or 0 if no such sequence exists.

Example 1:

``````1
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Input: beginWord = "hit", endWord = "cog", wordList = ["hot","dot","dog","lot","log","cog"]
Output: 5
Explanation: One shortest transformation sequence is "hit" -> "hot" -> "dot" -> "dog" -> cog", which is 5 words long.
``````

Example 2:

``````1
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Input: beginWord = "hit", endWord = "cog", wordList = ["hot","dot","dog","lot","log"]
Output: 0
Explanation: The endWord "cog" is not in wordList, therefore there is no valid transformation sequence.
``````

Constraints:

• 1 <= beginWord.length <= 10
• endWord.length == beginWord.length
• 1 <= wordList.length <= 5000
• wordList[i].length == beginWord.length
• beginWord, endWord, and wordList[i] consist of lowercase English letters.
• beginWord != endWord
• All the words in wordList are unique.

## Sample C++ Code

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class Solution {
public:
int ladderLength(string beginWord, string endWord, vector<string>& wordList) {
int steps = 0;
unordered_map<string, int> visited;

for (int i = 0; i < wordList.size(); i ++) {
visited[wordList[i]] = 0;
}
if (visited.find(endWord) == visited.end())
return steps;

queue<string> bfsQ;
bfsQ.push(beginWord);
int l;
string curr, temp;
bool found = false;
while (!bfsQ.empty()) {
l = bfsQ.size();
for (int i = 0; i < l; i ++) {
curr = bfsQ.front();
bfsQ.pop();
if (curr == endWord) {
found = true;
break;
}
for (int j = 0; j < curr.size(); j ++) {
temp = curr;
for (char c = 'a'; c <= 'z'; c ++) {
temp[j] = c;
if (visited.find(temp) != visited.end()) {
if (visited[temp] == 0) {
bfsQ.push(temp);
visited[temp] = 1;
}
}
}
}
}
steps ++;
if (found)
break;
}
if (found)
return steps;
else
return 0;
}
};
``````