226_翻转二叉树.md

import java.util.*;

/**
 * @author : xfhy
 * Create time : 2020年10月21日08:56:09
 * Description : 226. 翻转二叉树
 * source : https://leetcode-cn.com/problems/invert-binary-tree/
 */
public class Solution {

    public static class TreeNode {
        int val;
        TreeNode left;
        TreeNode right;

        TreeNode() {
        }

        TreeNode(int val) {
            this.val = val;
        }

        TreeNode(int val, TreeNode left, TreeNode right) {
            this.val = val;
            this.left = left;
            this.right = right;
        }
    }

    /**
     * 思路: 直接将左右子树交换即可,递归下去
     */
    public static TreeNode invertTree(TreeNode root) {
        invertHelper(root);
        return root;
    }

    private static void invertHelper(TreeNode root) {
        if (root == null) {
            return;
        }
        TreeNode temp = root.left;
        root.left = root.right;
        root.right = temp;
        invertHelper(root.left);
        invertHelper(root.right);
    }

    public static TreeNode createBinaryTree(LinkedList<Integer> inputList) {
        if (inputList == null || inputList.isEmpty()) {
            return null;
        }
        TreeNode node = null;
        Integer data = inputList.removeFirst();
        if (data != null) {
            node = new TreeNode(data);
            node.left = createBinaryTree(inputList);
            node.right = createBinaryTree(inputList);
        }
        return node;
    }

    public static void main(String[] args) {
        /*LinkedList<Integer> integers = new LinkedList<>(Arrays.asList(5, 4, 11, 7, null, null, 2, null, null, null, 8, 13, null, null, 4,
                5, null, null, 1));*/
        /*LinkedList<Integer> integers = new LinkedList<>(Arrays.asList(3, 2, 9, null, null, 10, null,
                null, 8, null, 4));*/
        LinkedList<Integer> integers = new LinkedList<>(Arrays.asList(7, 3, null, null, 15, 9, null, null, 20));
        TreeNode binaryTree = createBinaryTree(integers);

        TreeNode treeNode = invertTree(binaryTree);
        System.out.println(treeNode);
    }

}