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.

使用先序遍历迭代方式，直接代码如下：

/**
 * 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) {  //使用线序遍历的迭代方式
        if( !root ) return;
        stack<TreeNode*> qt;
        qt.push(root); 
        TreeNode* pre = 0;  //记录前驱节点
        while( !qt.empty() ) {
            TreeNode* cur = qt.top();
            qt.pop();
            if( cur->right ) qt.push(cur->right);   //先放右子树
            if( cur->left ) qt.push(cur->left);     //再放左子树
            if( pre ) { //如果前驱节点不为空的情况下
                pre->right = cur;
                pre->left = 0;
            }
            pre = cur;  //更新前驱节点
        }
        pre->right = 0;
        pre->left = 0;
    }
};