leetcode 792. Number of Matching Subsequences

Given string S and a dictionary of words words, find the number of words[i] that is a subsequence of S.

Example :
Input: 
S = "abcde"
words = ["a", "bb", "acd", "ace"]
Output: 3
Explanation: There are three words in words that are a subsequence of S: "a", "acd", "ace".
Note:

All words in words and S will only consists of lowercase letters.
The length of S will be in the range of [1, 50000].
The length of words will be in the range of [1, 5000].
The length of words[i] will be in the range of [1, 50].

思路:先用vector记录每个字母出现的位置，然后遍历每个单词，如果每个字母的位置是递增的那么这个单词就符合要求。如何判断位置见的递增关系呢？可以用二分实现。具体看代码

class Solution {
public:
    int numMatchingSubseq(string S, vector<string>& words) {
        vector<int> v[26];
        for (int i = 0; i < S.size(); ++i) {
            v[S[i]-'a'].emplace_back(i);
        }
        int ans = 0;
        for (auto i : words) {
            int mark = 0;
            int y = -1;
            for(auto c : i) {
                int x = c - 'a';
                if (v[x].empty()) {
                    mark = 1; break;
                }
                int pos = upper_bound(v[x].begin(), v[x].end(), y) - v[x].begin();
                if (pos < v[x].size()) {
                    y = v[x][pos];
                } else {
                    mark = 1; break;
                }
            }
            if (!mark) ans++;
        }
        return ans;
    }
};