Java O(n2) memory search

  • 0

    Draw the search tree and you'll find many subtree repeated:

    class Solution {
        public boolean checkValidString(String s) {
            if(s==null) return false;
            if(s.length()==0)return true;
            return dfs(0, 0, s.toCharArray(), new int[s.length()][s.length()+1]);
        private boolean dfs(int idx, int cnt, char[] sArr, int[][] memo){
            if(cnt < 0) return false;
            if(idx >= sArr.length){
                return cnt==0;
            if(memo[idx][cnt] != 0){
                return memo[idx][cnt]==2; // 2 means valid 1 invalid
            boolean ret = false;
                ret = dfs(idx+1, cnt+1, sArr, memo);
            }else if(sArr[idx]==')'){
                ret = dfs(idx+1, cnt-1, sArr, memo);
                ret = dfs(idx+1, cnt, sArr, memo)
                    ||dfs(idx+1, cnt-1, sArr, memo)
                    ||dfs(idx+1, cnt+1, sArr, memo);
            memo[idx][cnt] = ret?2:1;
            return ret;

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