【leetcode】114. Flatten Binary Tree to Linked List【java】

最新推荐文章于 2022-07-27 15:08:45 发布

陈善亮-BUPT

最新推荐文章于 2022-07-27 15:08:45 发布

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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。通过深度优先搜索（DFS）实现这一过程，使得每个节点的右子节点指向原二叉树中下一个节点的预序遍历顺序。

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

For example,
Given

The flattened tree should look like:

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.

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
//DFS:使用栈
/*public class Solution {
    public void flatten(TreeNode root) {
        if (root == null) {
            return;
        }
        Stack<TreeNode> stack = new Stack<TreeNode>();
        stack.push(root);
        while (!stack.isEmpty()) {
            TreeNode cur = stack.pop(); 
            if (cur.right != null) {
                stack.push(cur.right);
            }
            if (cur.left != null) {
                stack.push(cur.left);
            }
            if (!stack.isEmpty()) {
                cur.right = stack.peek();
            }
            cur.left = null;
        }
    }
}*/

public class Solution {
    private TreeNode lastNode = null;
    public void flatten(TreeNode root) {
        if (root == null) {
            return;
        }
        if (lastNode != null) {
            lastNode.left = null;
            lastNode.right = root;
        }
        lastNode = root;
        TreeNode left = root.left;
        TreeNode right = root.right;
        flatten(left);
        flatten(right);
    }
}