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"

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

本題和LeetCode 127. Word Ladder —— 單詞接龍完全一樣，沒有任何區別，利用廣度優先搜索查找最短路徑。欣慰自己寫代碼一遍過，直接AC。

Java

class Solution {
    public int minMutation(String start, String end, String[] bank) {
        if (bank == null || bank.length == 0) return -1;
        char[] gen = {‘A‘,‘C‘,‘G‘,‘T‘};
        HashSet<String> bankSet = new HashSet<>();
        for (String s : bank)
            bankSet.add(s);
        Queue<String> q = new LinkedList<>();
        HashMap<String, Integer> res = new HashMap<>();
        res.put(start, 0);
        q.add(start);
        while (!q.isEmpty()) {
            String s = q.poll();
            bankSet.remove(s);
            for (int i = 0; i < s.length(); i++) {
                char[] next = s.toCharArray();
                for (char c : gen) {
                    next[i] = c;
                    String nextS = new String(next);
                    if (bankSet.contains(nextS)) {
                        res.put(nextS, res.get(s) + 1);
                        if (nextS.equals(end)) return res.get(nextS);
                        q.add(nextS);
                    }
                }
            }
        }
        return -1;
    }
}

(Java) LeetCode 433. Minimum Genetic Mutation —— 最小基因變化