题目大意：给你一个基因库，问一个基因最少需要变异多少次才能变为另外一个基因。每次变异只能修改一个字符，并且必须在基因库里。

https://leetcode.com/problems/minimum-genetic-mutation/description/

A gene string can be represented by an 8-character long string, with choices from "A", "C", "G", "T".

Suppose we need to investigate about a mutation (mutation from “start” to “end”), where ONE mutation is defined as ONE single character changed in the gene string.

For example, "AACCGGTT" -> "AACCGGTA" is 1 mutation.

Also, there is a given gene “bank”, which records all the valid gene mutations. A gene must be in the bank to make it a valid gene string.

Now, given 3 things – start, end, bank, your task is to determine what is the minimum number of mutations needed to mutate from “start” to “end”. If there is no such a mutation, return -1.

Note:

1. Starting point is assumed to be valid, so it might not be included in the bank.
2. If multiple mutations are needed, all mutations during in the sequence must be valid.
3. You may assume start and end string is not the same.

Example 1:

start: "AACCGGTT"
end:   "AACCGGTA"
bank: ["AACCGGTA"]

return: 1

Example 2:

start: "AACCGGTT"
end:   "AAACGGTA"
bank: ["AACCGGTA", "AACCGCTA", "AAACGGTA"]

return: 2

Example 3:

start: "AAAAACCC"
end:   "AACCCCCC"
bank: ["AAAACCCC", "AAACCCCC", "AACCCCCC"]

return: 3

Solution: BFS Shortest Path

Time complexity: O(n^2)

Space complexity: O(n)

// Author: Huahua
// Running time: 4 ms
class Solution {
public:
  int minMutation(string start, string end, vector<string>& bank) {    
    queue<string> q;
    q.push(start);
    
    unordered_set<string> visited;
    visited.insert(start);
    
    int mutations = 0;
    while (!q.empty()) {
      size_t size = q.size();
      while (size--) {
        string curr = std::move(q.front()); q.pop();
        if (curr == end) return mutations;
        for (const string& gene : bank) {
          if (visited.count(gene) || !validMutation(curr, gene)) continue;
          visited.insert(gene);
          q.push(gene);
        }
      }
      ++mutations;
    }    
    return -1;
  }
private:
  bool validMutation(const string& s1, const string& s2) {
    int count = 0;
    for (int i = 0; i < s1.length(); ++i)
      if (s1[i] != s2[i] && count++) return false;
    return true;
  }
};

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