392. Is Subsequence

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?

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

 

Approach #1:

class Solution {
public:
    bool isSubsequence(string s, string t) {
        int len1 = s.length();
        int len2 = t.length();
        if (len1 == 0) return true;
        queue<char> q;
        for (int i = 0; i < len1; ++i) {
            q.push(s[i]);
        }
        for (int i = 0; i < len2; ++i) {
            if (q.front() == t[i]) {
                q.pop();
            }
            if (q.empty()) return true;
        }
        return false;
    }
};

Runtime: 52 ms, faster than 64.57% of C++ online submissions for Is Subsequence.

 

Approach #2: Binary Serach

class Solution {
public:
    bool isSubsequence(string s, string t) {
        vector<vector<int>> temp(26);
        for (int i = 0; i < t.length(); ++i)
            temp[t[i]-'a'].push_back(i);
        int index = -1;
        for (int i = 0; i < s.length(); ++i) {
            auto it = upper_bound(temp[s[i]-'a'].begin(), temp[s[i]-'a'].end(), index);
            if (it == temp[s[i]-'a'].end()) return false;
            index = *it;
        }
        return true;
    }
};

Runtime: 52 ms, faster than 64.57% of C++ online submissions for Is Subsequence.