# [LeetCode] Is Subsequence 是子序列

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.

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.

class Solution {
public:
bool isSubsequence(string s, string t) {
int i = 0;
for (int j = 0; j < t.size() && i < s.size(); ++j) {
if (s[i] == t[j]) ++i;
}
return i == s.size();
}
};

// Follow up
class Solution {
public:
bool isSubsequence(string s, string t) {
int pre = -1, n = t.size();
unordered_map<char, vector<int>> char2pos;
for (int i = 0; i < n; ++i) char2pos[t[i]].push_back(i);
for (char c : s) {
auto it = upper_bound(char2pos[c].begin(), char2pos[c].end(), pre);
if (it == char2pos[c].end()) return false;
pre = *it;
}
return true;
}
};

Number of Matching Subsequences

https://leetcode.com/problems/is-subsequence/