Is Subsequence - leetcode 392号题目个人题解

题目要求

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?

解题思路

这道题老师讲解的时候使用了动态规划的思想，但其实这道题不用动态规划的思想也能在相同的时间复杂度的情况下解出来。
我们设置两个标签:index1,index2。
初始状态时index1指向s字符串的最左边一位。
index2在t字符串中从左往右扫，若找到一个与index1所指向的字母相同的字母，就把index1向右移一位。
直到index2把t字符串扫完或者s字符串中的字母被全部找到。
代码实现如下：

代码实现

class Solution {
public:
bool isSubsequence(string s, string t) {
    if(s.empty()) return true;
    if(t.empty()) return false;
    int index1=0;
    int index2=0;
    while(index2<t.size()){
        if(s[index1]==t[index2]) index1++;
        if(index1==s.size()) return true;
        index2++;
    }
    return false;
}
};

复杂度分析

该算法的复杂度应该是O(n)的。