struct TreeNode * stack[1024],* tmp;
 int maxDepth(struct TreeNode * root)
 {
     int top = 0,ans = 0,val;
     struct TreeNode * left ,* right;
     if(root == NULL) return 0;
     stack[top++] = root;
     root->val = 1;
     while(top)
     {
         tmp = stack[--top];
         val = tmp->val;
         left = tmp ->left;
         right = tmp -> right;
         if(val > ans)
            ans = val;
         if(left)
         {
            stack[top++] = left;
            left->val = val+1;
         }
         if(right)
         {
            stack[top++] = right;
            right->val = val+1;
         }
     }
     return ans;
 }

int maxDepth2(struct TreeNode* root) {
    if(root == NULL ) return 0;
    int a = maxDepth(root->left);
    int b = maxDepth(root->right);
    return (a>b?a:b)+1;
}