392. Is Subsequence

• Difficulty: Easy

• Topics: Binary Search, Dynamic Programming, Greedy

• Similar Questions:

  • Number of Matching Subsequences

  • Shortest Way to Form String

Problem:

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). <="" p="">

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?

Credits:
Special thanks to @pbrother for adding this problem and creating all test cases.

Solutions:

class Solution {
public:
    bool isSubsequence(string s, string t) {
        if (s.length() > t.length())    return false;
        int pos = 0;
        for (int i = 0; i < s.length(); ++i) {
            while (pos < t.length() && t[pos] != s[i]) {
                ++pos;
            }
            if (pos == t.length())  return false;
            ++pos;
        }

        return true;
    }
};