Recover Binary Search Tree

 1 /**
 2  * Definition for binary tree
 3  * struct TreeNode {
 4  *     int val;
 5  *     TreeNode *left;
 6  *     TreeNode *right;
 7  *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 8  * };
 9  */
10 class Solution {
11 public:
12     void recoverTree(TreeNode *root) {
13         if( !root ) return ;
14         stack<TreeNode*> st;
15         TreeNode* cur = root;
16         while( cur ) {
17             st.push(cur);
18             cur = cur->left;
19         }
20         TreeNode* pre = 0;
21         TreeNode* fnode = 0;
22         TreeNode* snode = 0;
23         while( !st.empty() ) {
24             cur = st.top();
25             st.pop();
26             if( pre && pre->val > cur->val ) {  //前驱值大于当前节点值时则为异常情况
27                 if( !fnode ) fnode = pre, snode = cur;  //可能出现出错的两个节点刚好是前后连续的
28                 else {
29                     snode = cur;    //再次找到前驱值大于当前节点值，那么他们就不是相连的
30                     break;
31                 }
32             }
33             pre = cur;
34             cur = cur->right;
35             while( cur ) {
36                 st.push(cur);
37                 cur = cur->left;
38             }
39         }
40         swap(fnode->val, snode->val);   //交换两节点的值
41     }
42 };