题目描述:

Given a string s and a string t, check if s is subsequence of t.

You may assume that there is only lower case English letters in both s and t. t is potentially a very long (length ~= 500,000) string, and s is a short string (<=100).

A subsequence of a string is a new string which is formed from the original string by deleting some (can be none) of the characters without disturbing the relative positions of the remaining characters. (ie, "ace" is a subsequence of "abcde" while "aec" is not).

Example 1: s = "abc", t = "ahbgdc"

Return true.

Example 2: s = "axc", t = "ahbgdc"

Return false.

双指针, O(n)时间复杂度.

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class Solution {
public:
    bool isSubsequence(string s, string t) {
        int sLen = s.length(), tLen = t.length();
        if(sLen == 0 && tLen == 0) return true;
        int sp = 0;
        for(int i = 0; i < tLen; i++){
            if(s[sp] == t[i]) sp++;
            if(sp == sLen) return true;
        }
        return false;
    }
};