30. 串联所有单词的子串（滑动窗口）

class Solution {
    public List<Integer> findSubstring(String s, String[] words) {
        List<Integer> res = new ArrayList<>();
        int n = s.length(), m = words.length;
        if(n == 0 || m == 0) return res;
        int len = words[0].length();
        if(n < m * len) return res;
        Map<String,Integer> map = new HashMap<>();
        int sum = 0;
        for(String str : words) {
            if(!map.containsKey(str)) sum++;
            map.put(str,map.getOrDefault(str,0)+1);
        }
        int l = 0;
        while(l + m * len <= n) {
            Map<String,Integer> window = new HashMap<>();
            int count = 0;
            int r = l;
            String str = s.substring(r,r+len);
            while(map.containsKey(str)) {
                window.put(str,window.getOrDefault(str,0)+1);
                if(window.get(str) == map.get(str)) count++;
                if(window.get(str) > map.get(str)) break;
                if(count == sum) {
                    res.add(l);
                    break;
                }
                r += len;
                str = s.substring(r,r+len);
            }
            l++;
        }
        return res;
    }
}