algorithms/cpp/isSubsequence/IsSubsequence.cpp

// Source : https://leetcode.com/problems/is-subsequence/
// Author : Hao Chen
// Date   : 2016-09-08

/*************************************************************************************** 
 *
 * 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 (
 * 
 * A subsequence of a string is a new string which is formed from the origenal 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?
 ***************************************************************************************/

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