Leetcode 之Binary Tree Postorder Traversal（47）

void recoverTree(TreeNode *root)
      {
          pre = p = q = nullptr;
          dfs(root);
          swap(p->val, q->val);
      }
      void dfs(TreeNode *root)
      {
          if (!root)return;
          dfs(root->left);
          if (pre != nullptr && pre->val > root->val)
          {
              if (p == nullptr)
              {
                  p = pre;
                  q = root;
              }
              else
                  q = root;
          }
          pre = root;
          dfs(root->right);
      }