leetcode114~Flatten Binary Tree to Linked List

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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

public class FlattenTree {//使用栈实现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()){TreeNode tmp = stack.pop();node.right=tmp;}node = node.right;}}//前序遍历二叉树，遇到左子节点时，将其变为右子节点，并将左置空//因此需要一个全局变量存储之前的右子节点private TreeNode lastNode = null;public void flatten2(TreeNode root) {if(root==null) return;if(lastNode !=null) {lastNode.left = null;lastNode.right = root;}lastNode = root;TreeNode right = root.right;flatten2(root.left);flatten2(right);}
}

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