392. Is Subsequence

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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.

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?

解法1：O(N) 解法

用两个指针一前一后的分别指向string的位置。如果两个字符相等就同前进，如果不相等，那么只前进target的指针，因为subsequence可以要求是不连续的。
然后判断一下s的指针时候到尾巴了就可以了。如果t到尾巴了还没有结果，则说明答案是false。
C++

1

Java

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public class Solution {
    public boolean isSubsequence(String s, String t) {
        if ((s == null && t == null) || (s.length() == 0)) {
            return true;
        }
        if (s.length() > t.length()) {
            return false;
        }
        int i = 0, j = 0;
        while (j < t.length()) {
            if (t.charAt(j) == s.charAt(i)) {
                i++;
                if (i == s.length()) return true;
            }
            j++;
        }
        return false;
    }
}