LeetCode - Is Subsequence

Question Definition

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?

Java Solution

public boolean isSubsequence(String s, String t) {
    List<Integer>[] idx = new List[256]; // Just for clarity
    for (int i = 0; i < t.length(); i++) {
        if (idx[t.charAt(i)] == null)
            idx[t.charAt(i)] = new ArrayList<>();
        idx[t.charAt(i)].add(i);
    }

    int prev = 0;
    for (int i = 0; i < s.length(); i++) {
        if (idx[s.charAt(i)] == null) return false; // Note: char of S does NOT exist in T causing NPE
        int j = Collections.binarySearch(idx[s.charAt(i)], prev);
        if (j < 0) j = -j - 1;
        if (j == idx[s.charAt(i)].size()) return false;
        prev = idx[s.charAt(i)].get(j) + 1;
    }
    return true;
}