【leetcode】Recover Binary Search Tree

Recover Binary Search Tree

Two elements of a binary search tree (BST) are swapped by mistake.

Recover the tree without changing its structure.

Note:
A solution using O(n) space is pretty straight forward. Could you devise a constant space solution?

中序遍历解法O(n)space

 /**
  * Definition for binary tree
  * struct TreeNode {
  *     int val;
  *     TreeNode *left;
  *     TreeNode *right;
  *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
  * };
  */
 class Solution {
 public:
     void recoverTree(TreeNode *root) {
         vector<TreeNode *> nodes;
         dfs(root,nodes);
         int index1=-;
         int index2=-;
         for(int i=;i<nodes.size();i++)
         {
             if(nodes[i]->val<nodes[i-]->val)
             {
                 if(index1==-) index1=i-;
                 index2=i;
             }
         }

         int tmp=nodes[index1]->val;
         nodes[index1]->val=nodes[index2]->val;
         nodes[index2]->val=tmp;
         return;

     }     

     void dfs(TreeNode *root,vector<TreeNode *> &nodes)
     {
         if(root==NULL) return;
         dfs(root->left,nodes);
         nodes.push_back(root);
         dfs(root->right,nodes);
     }
 };

直接在中序遍历中进行判断，需要记录遍历中的前一个元素

如果前一个元素比当前元素大，则说明存在错误

 /**
  * Definition for binary tree
  * struct TreeNode {
  *     int val;
  *     TreeNode *left;
  *     TreeNode *right;
  *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
  * };
  */
 class Solution {
 public:
     void recoverTree(TreeNode *root) {

         TreeNode *firstError=NULL;
         TreeNode *secondError=NULL;
         TreeNode *pre=NULL;

         dfs(root,pre,firstError,secondError);

         int tmp=firstError->val;
         firstError->val=secondError->val;
         secondError->val=tmp;
     }

     void dfs(TreeNode *root,TreeNode *&pre,TreeNode *&firstError,TreeNode *&secondError)
     {
         if(root==NULL) return;

         dfs(root->left,pre,firstError,secondError);

         if(pre!=NULL&&pre->val>root->val)
         {
             if(firstError==NULL) firstError=pre;
             secondError=root;
         }
         pre=root;

         dfs(root->right,pre,firstError,secondError);
     }
 };