日拱算法：最长字符串链，什么是“词链”？

输入：words = ["a","b","ba","bca","bda","bdca"]
输出：4
解释：最长单词链之一为 ["a","ba","bda","bdca"]

输入：words = ["xbc","pcxbcf","xb","cxbc","pcxbc"]
输出：5
解释：所有的单词都可以放入单词链 ["xb", "xbc", "cxbc", "pcxbc", "pcxbcf"].

var longestStrChain = function(words) {
    words.sort((a, b) => a.length - b.length);
    const M = new Map();
    let res = 1;
    for (let s of words) {
        if (M.has(s)) continue;  // 重复
        M.set(s, 1);
        for (let i = 0; i < s.length; i++) {
            if (i > 0 &amp;&amp; s[i] === s[i - 1]) continue;  // 重复
            let t = s.substr(0, i) + s.substr(i + 1);
            if (M.has(t) &amp;&amp; M.get(t) >= M.get(s)) {
                M.set(s, M.get(t) + 1);
            }
        }
        res = Math.max(res, M.get(s));
    }
    return res;
};