void delete_zero(char* s) {     int len;     int i;     len = strlen(s);     for (i = len; i >= 0; i--)     {         if (s[i] == '0')             len--;         else              break;

Given n, how many structurally unique BST's (binary search trees) that store values 1...n? For example, Given n = 3, there are a total of 5 unique BST's. 1 3 3 2 1