31. Flatten Binary Tree to Linked List

Flatten Binary Tree to Linked List

Given a binary tree, flatten it to a linked list in-place.

For example, Given

         1
        / 
       2   5
      /    
     3   4   6

The flattened tree should look like:

   1
    
     2
      
       3
        
         4
          
           5
            
             6

click to show hints.

Hints:

If you notice carefully in the flattened tree, each node's right child points to the next node of a pre-order traversal

思想： 如 Hints.

/**
 * Definition for binary tree
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 * };
 */
 TreeNode* getPreNode(TreeNode *root) {
     if(root == NULL) return NULL;
     while(root->right)  root = root->right;
     return root;
 }
class Solution {
public:
    void flatten(TreeNode *root) {
        if(root == NULL) return;
        TreeNode *pre = getPreNode(root->left);
        if(pre) {
         pre->right = root->right;
         root->right = root->left;
         root->left = NULL;
        }
        flatten(root->right);
    }
};