392. Is Subsequence

Description

A subsequence of a string is a new string that is formed from the original string by deleting some (can be none) of the characters without disturbing the relative positions of the remaining characters. (i.e., "ace" is a subsequence of "abcde" while "aec" is not).

Example 1:

Input: s = "abc", t = "ahbgdc"
Output: true

Idea

Use two different index as two pointers, first one represents s pointer (like s[index]), second one represents t pointer (like t[index]).

Then we want to analyse if s is the subsequence of t by check their content one by one: For instance, s = “abc”, t = “ahbgdc”, s[0] is “a” equals to t[0], so we increase the index and check the next letter s[1] and t[1]; now t[1] doesn’t equals to s[1], so increase t’s index and check if next letter in t equals to s[1].

Code

class Solution {
public:
    bool isSubsequence(string s, string t) {
          // initialize two pointers
        int s_index = 0;
        int t_index = 0;
        // Check the letter one by one
        while ( t_index < t.size() ) {
            if ( s[s_index] == t[t_index]) {
                s_index++;
                t_index++;
            } else {
                t_index++;
            }
        }
        if ( s_index == s.size()) {
            return true;
        } else {
            return false;
        }
    }
};

---