Is Subsequence

-問題描述-
-演算法分析-
-程式碼實現-

問題描述

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?

演算法分析

問題大意為給出兩個字串s,t，判斷s是否為t的子序列。
演算法分析：
如果s[i] == t[j]，那麼可以判斷s中下一個字元是否在t中存在，即i++,j++;
如果s[i] != t[j],那麼可以繼續向下判斷t中是否存在這個字元，即j++；
如果最終s中的所有字元在t中均存在，那麼s是t的子序列。

程式碼實現

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