392. Is Subsequence

Link

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?

Ans: Instead of using indexOf() funciton to find the index in t

• Create a hashMap<Character, List<Integer>> to store the indexs of each char in t

• write a binary search for finding the index of s.charAt(t) in list .

TimeComplexity: O(klogn), where k is the s.length, and n is the t.length

Solution

IndexOf cost O(n), where n is the length of t

Time complexity : O(kn)

  public static boolean isSubsequence(String s, String t) {
        int previousIndexInT = 0;
        for(int i = 0; i < s.length(); i++){
            int indexInT = t.indexOf(s.charAt(i), previousIndexInT);
            if(indexInT == -1){
                return false;
            }
            previousIndexInT = indexInT + 1; 
            //REMEMBER 要從下一個開始找，不然會一直在同一個
        }
        return true;
    }