392. Is Subsequence

难度: Easy

题目描述

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?

解题思路

代码实现

解决方案

java

class Solution {
    public  static final Map<String,Map<Character, TreeSet<Integer>>> tMap = new HashMap<>();


    public boolean isSubsequence(String s, String t) {
        if("".equals(s)){
            return true;
        }
        Map<Character, TreeSet<Integer>> map = tMap.get(t);
        if(map == null){
            map = new HashMap<>();
            for(int i = 0; i < t.length(); i++) {
                char c = t.charAt(i);
                if(!map.containsKey(c)) {
                    map.put(c, new TreeSet<>());
                }
                map.get(c).add(i);
            }
            tMap.put(t, map);
        }

        Integer preIndex = t.indexOf(s.charAt(0));
        if (preIndex < 0) {
            return false;
        }
        for (int i = 1; i < s.length(); i++) {
            char c = s.charAt(i);
            if (!map.containsKey(c)) {
                return false;
            }
            TreeSet<Integer> set = map.get(c);
            Integer targetData = set.higher(preIndex);
            if (targetData == null) {
                return false;
            }
            preIndex = targetData;
        }
        return true;
    }
}