LeetCode: 392. Is Subsequence

 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 "abcde" 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.

 

Follow up: Suppose there are lots of incoming s, say s1, s2, ..., sk where k >= 109, 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:
	/* Follow Up
	- k is big
	Time: O(k * len(s) * log len(t))
	Space: O(len(t) * len(t))
	*/
	bool isSubsequence(string s, string t) {
	// preprocess: "abb" -> {a: [0], b: [1, 2]}
	map<char, vector<int>> indexes;
	for(int i = 0; i < t.size(); i++)
	indexes[t[i]].push_back(i);
	// check each s
	int cur = -1;
	for(auto c: s){
	// char not found in t
	if(indexes.find(c) == indexes.end())
	return false;
	// binary search
	// find the first pos whose value is greater than prev
	auto idx = indexes[c];
	auto it = upper_bound(idx.begin(), idx.end(), cur);
	if(it == idx.end())
	return false;
	cur = *it;
	}
	return true;
	}
	};

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