step1 :
    dp functon!


step2:

1: if return dp[n][m] //compare with other, compare two !

    for(int j = 1; j <= n; j ++)
        for(int i = 1; i < i; i ++)


2: if return dp[1][n] // if compare with itself

    for(int len = 0; len <= n; len ++)
        for(int i = 0, j = len; j <= n; i ++, j ++)