【LeetCode】114. 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.

前序遍历，按访问序往右加就行了。

需要注意的是，加入单链之后要删掉原先的子女链接。

/**
 * 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 flatten(TreeNode *root) {
        if(!root)
            return;

        TreeNode* newtail = new TreeNode(-1);
        stack<TreeNode*> stk;
        stk.push(root);
        while(!stk.empty())
        {
            TreeNode* top = stk.top();
            newtail->right = top;
            newtail = newtail->right;
            stk.pop();
            
            if(top->right)
                stk.push(top->right);
            if(top->left)
                stk.push(top->left);
                
            top->left = NULL;
            top->right = NULL;
        }
    }
};