114. Flatten Binary Tree to Linked List

题目：

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.

链接： http://leetcode.com/problems/flatten-binary-tree-to-linked-list/

题解：

使用栈来辅助，很像先序遍历。也可以用recursive和Morris-travel。

Time Complexity - O(n)， Space Complexity - O(n)。

public class Solution {
public void flatten(TreeNode root) {
if(root == null)
return;
Stack<TreeNode> stack = new Stack<TreeNode>();
TreeNode node = root;

while(node != null || !stack.isEmpty()){
if(node.right != null){
stack.push(node.right);
}
if(node.left != null){
node.right = node.left;
node.left = null;
} else {
if(!stack.isEmpty())
node.right = stack.pop();
}
node = node.right;
}
}
}

测试：