Flatten Binary Tree to Linked List

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本文介绍了一种将二叉树结构通过特定算法展平为链表的方法。通过迭代和递归两种方式实现，使得每个节点的右子节点指向二叉树前序遍历的下一个节点。

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

For example,
Given

The flattened tree should look like:

Hints:

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

/**
 * Definition for binary tree
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
public class Solution {
    public void flatten(TreeNode root) {
		// Start typing your Java solution below
		// DO NOT write main() function
		if (root == null)
			return;
		Stack<TreeNode> stack = new Stack<TreeNode>();
		stack.push(root);
		TreeNode node = null;
		while (!stack.isEmpty()) {
			TreeNode cur = stack.pop();
			if (node != null) {
				node.left = null;
				node.right = cur;
			}
			node = cur;
            if (cur.right != null) {
				stack.push(cur.right);
			}
			if (cur.left != null) {
				stack.push(cur.left);
			}
		}
	}
}

A recursive solution.

/**
 * Definition for binary tree
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
public class Solution {
    TreeNode res = new TreeNode(0);
	Stack<TreeNode> stk = new Stack<TreeNode>();
	public void flatten(TreeNode root){
		// Start typing your Java solution below
		// DO NOT write main() function
		if(root == null)
			return;
		if(root != null){
			res.left = null;
			res.right = root;
            res = root;
		}
		if (root.right != null) {
			stk.push(root.right);
		}
		if (root.left != null) {
			stk.push(root.left);
		}
        if(!stk.isEmpty()){
		    flatten(stk.pop());
	    }
	}
}