3. Longest Substring Without Repeating Characters

JAVA刷题ing

Medium

Given a string, find the length of the longest substring without repeating characters.

Example 1:

Input: "abcabcbb"
Output: 3 
Explanation: The answer is "abc", with the length of 3.

Example 2:

Input: "bbbbb"
Output: 1
Explanation: The answer is "b", with the length of 1.

Example 3:

Input: "pwwkew"
Output: 3
Explanation: The answer is "wke", with the length of 3. 
             Note that the answer must be a substring, "pwke" is a subsequence and not a substring.

1. two point

class Solution {
    public int lengthOfLongestSubstring(String s) {
        Set<Character> mem = new HashSet<Character>();
        int i = 0; int j = 0;
        int n = s.length();
        int ans = 0;
        while(i < n && j < n){
            if(!mem.contains(s.charAt(j))){
                
                mem.add(s.charAt(j++)); // after finishing this, j += 1
                ans = Math.max(ans, j-i); 
                // this why not j-i+1, because at this time j already plus 1
                // System.out.println("here"+mem+s.charAt(j-1));
            }else{
                // we move i until find the same element
                mem.remove(s.charAt(i++));
                // System.out.println("d"+mem+s.charAt(j));
            }
        }
        return ans;
    }
}

Syntax:

Set<Character> mem = new HashSet<Character>(); // correct
HashSet<Character> mem = new HashSet<Charater>(); // correct
Set<Character> mem = new Set<Character>(); // wrong Set is abstract; cannot be instantiated

1. 关于Hashmap的声明，为什么前面可以是Set后面必须是HashSet，这里。总之，Set包括(HashSet, LinkedHashSet or TreeSet)前面是对对象类型的声明，类型可以是总的，或者是具体的，但后面是调用接口，对对象初始化，这里就没有set这一选项了。

System.out.println("d"+mem+s.charAt(j));

2. debug必备。print don’t change the line, but println (means print line next) change line after printing. using ‘+’ to link two string. ‘ ‘ can only be used to one char, “ “ can be used to string.