LeetCode解题报告 392. Is Subsequence [medium]

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

.

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

由于子串不需要连续，只要保持位置不打乱原来的顺序就可以，因此用两个指针，一个指向原数列，一个指向目标子串即可，遍历一遍原数列可得到结果。

时间复杂度分析

遍历一遍字符串t可得结果，因此时间复杂度为O(n)。

代码如下：

class Solution {
public:
bool isSubsequence(string s, string t) {
if (s.length()>t.length()) {
return false;
}
else{
int l1=s.length();
int l2=t.length();
int k=0;
for (int i=0; i<l2; i++) {
if (s[k]==t[i]) {
k++;
}
}
if (k==l1) {
return true;
}
else
return false;
}
}
};