树-对称二叉树的学习

一、思路

对称二叉树有两种写法：迭代法和递归法；
两种方法都运用了双指针；
个人认为迭代法更好理解一些；
写出了迭代之后，写出递归相对容易了一些；
递归最重要的是终止条件；

二、迭代写法

import java.util.LinkedList;
import java.util.Queue;

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

    // 构造函数
    TreeNode() {}
    TreeNode(int val)
    { this.val = val; }

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

public class Solution {
    public boolean isSymmetric(TreeNode root){
        // 队列存放每两个位置存放
        Queue<TreeNode> queue = new LinkedList<>();
        TreeNode p,q;
        queue.add(root);
        queue.add(root);
        while(!queue.isEmpty()){
            p = queue.poll();
            q = queue.poll();
            if(p == null && q == null)continue;
            if(p.val != q.val || p == null || q == null){
                return false;
            }
            // 如果 p 和 q 是空，则跳过增加其子树阶段
            queue.add(p.left);
            queue.add(q.right);
            queue.add(p.right);
            queue.add(q.left);
        }
        return true;
    }
}

class test{
    public static void main(String[] args) {
        TreeNode node1 = new TreeNode(1);
        TreeNode node2 = new TreeNode(2);
        TreeNode node3 = new TreeNode(2);
        TreeNode node4 = new TreeNode(4);
        TreeNode node5 = new TreeNode(5);
        TreeNode node6 = new TreeNode(5);
        TreeNode node7 = new TreeNode(4);
        node1.left = node2;node1.right = node3;
        node2.left = node4;node2.right = node5;
        node3.left = node6;node3.right = node7;
        System.out.println(new Solution().isSymmetric(node1));
    }
}

三、递归写法

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

    // 构造函数
    TreeNode() {}
    TreeNode(int val)
    { this.val = val; }

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

class Solution {
    public boolean isSymmetric(TreeNode root){
        return judgeSymmetric(root,root);
    }

    public boolean judgeSymmetric(TreeNode p, TreeNode q){
        if(p == null && q == null)return true;
        else if(p == null || q == null || p.val != q.val)return false;
        else{
            return judgeSymmetric(p.left,q.right)&& judgeSymmetric(p.right,q.left);
        }
    }
}

class test{
    public static void main(String[] args) {
        TreeNode node1 = new TreeNode(1);
        TreeNode node2 = new TreeNode(2);
        TreeNode node3 = new TreeNode(2);
        TreeNode node4 = new TreeNode(4);
        TreeNode node5 = new TreeNode(5);
        TreeNode node6 = new TreeNode(5);
        TreeNode node7 = new TreeNode(4);
        node1.left = node2;node1.right = node3;
        node2.left = node4;node2.right = node5;
        node3.left = node6;node3.right = node7;
        System.out.println(new Solution().isSymmetric(node1));
    }
}