3. 无重复字符的最长子串

//滑动窗口法
class Solution {
    public int lengthOfLongestSubstring(String s) {
        int l = s.length();
        if(l == 1) return 1;
        int max = 0;
        Map<Character, Integer> map = new HashMap<>();
        int first = 0, second = 0;
        while(first < l && second < l){
            Character c = s.charAt(second);
            if(!map.containsKey(c)){
                map.put(c, second);
            }else{
                int index = map.get(c);
                //注意这块儿要控制子串头指针不能跳到前面去
                first = Math.max(first, index + 1);
                map.put(c, second);
            }
            second++;
            max = Math.max(second - first, max);
        }
        return max;
    }
}

改进，用数组代替map存索引

class Solution {
    public int lengthOfLongestSubstring(String s) {
        int l = s.length();
        if(l == 1) return 1;
        int max = 0;
        int[] indexArray = new int[128];
        for(int i = 0; i < 128; i++){
            indexArray[i] = -1;
        }
        int first = 0, second = 0;
        while(first < l && second < l){
            int c = s.charAt(second);
            if(indexArray[c] == -1){
                indexArray[c] = second;
            }else{
                int index = indexArray[c];
                first = Math.max(first, index + 1);
                indexArray[c] = second;
            }
            second++;
            max = Math.max(second - first, max);
        }
        return max;
    }
}