392.Is Subsequence.md

392. Is Subsequence

Given two strings s and t, return true if s is a subsequence of t, or false otherwise.

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 "<u>a</u>b<u>c</u>d<u>e</u>" while "aec" is not).

Example 1:

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

Example 2:

Input: s = "axc", t = "ahbgdc"
Output: false

Constraints:

• 0 <= s.length <= 100
• 0 <= t.length <= 104
• s and t consist only of lowercase English letters.`

Solution

Double pointers or DP.

Code

// double pointers
class Solution {
public:
    bool isSubsequence(string s, string t) {
        int n1 = s.size(), n2 = t.size();
        if (n1 > n2) return false;
        if (n1 == 0) return true;
        int left = 0, right = 0;
        while (left < n1 && right < n2) {
            if (s[left] == t[right]) {
                left++;
                right++;
            }
            else right++;
        }
        return left == n1;
    }
};

// DP
class Solution {
public:
    bool isSubsequence(string s, string t) {
        vector<vector<int>> dp(s.size() + 1, vector<int>(t.size() + 1, 0));
        for (int i = 1; i <= s.size(); i++) {
            for (int j = 1; j <= t.size(); j++) {
                if (s[i - 1] == t[j - 1]) dp[i][j] = dp[i - 1][j - 1] + 1;
                else dp[i][j] = dp[i][j - 1]; // delete t[j-1]
            }
        }
        if (dp[s.size()][t.size()] == s.size()) return true;
        return false;
    }
};