leetcode 392. Is Subsequence

392. Is Subsequence

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 sis 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.

Follow up:
If there are lots of incoming S, say S1, S2, ... , Sk where k >= 1B, and you want to check one by one to see if T has its subsequence. In this scenario, how would you change your code?

先写的递归。MLE。

class Solution {
public:
    bool isSubsequence(string s, string t) 
    {
        if (s == t || s.size() == 0) return true;
        else if (s.size() > t.size()) return false;
        if (s[0] == t[0] && isSubsequence(s.substr(1), t.substr(1))) return true;
        return isSubsequence(s, t.substr(1));
    }
};

然后发现自己想麻烦了，直接两个指针后移就可以。

class Solution {
public:
    bool isSubsequence(string s, string t) 
    {
        if (s == t || s.size() == 0) return true;
        else if (s.size() > t.size()) return false;
        
        int i = 0, j = 0;
        while (i < s.size() && j < t.size())
        {
            if (s[i] == t[j])
            {
                i++;
            }
            j++;
        }
        return i == s.size();
    }
};