101. 对称二叉树

题目描述

原题

给定一个二叉树,检查它是否是镜像对称的。

例如,二叉树 [1,2,2,3,4,4,3] 是对称的。

1
/ \
2 2
/ \ / \
3 4 4 3

但是下面这个 [1,2,2,null,3,null,3] 则不是镜像对称的:

1
/ \
2 2
\ \
3 3

进阶:

你可以运用递归和迭代两种方法解决这个问题吗?

题解

/**
* Definition for a binary tree node.
* public class TreeNode {
* int val;
* TreeNode left;
* TreeNode right;
* TreeNode(int x) { val = x; }
* }
*/
//这是我自己的解法
class Solution {
public boolean isSymmetric(TreeNode root) {
if(root == null) return true;
//层序遍历 存储到List中
//然后双指针校验回文数
LinkedList<TreeNode> queue = new LinkedList<>();
queue.offer(root);
while(!queue.isEmpty()){
int size = queue.size();
ArrayList<TreeNode> list = new ArrayList<>();
for(int i = 0;i < size;i++){
TreeNode node = queue.poll();
list.add(node);
if(node!=null){
queue.offer(node.left);
queue.offer(node.right);
}
}
//遍历数组
int i = 0,j = list.size() -1;
while(i<j){
TreeNode node1 =list.get(i);
TreeNode node2 =list.get(j);
if(node1!=null&&node2!=null){
if(node1.val != node2.val){
return false;
}
}else if(node1 != null || node2 != null){
return false;
}
i++;
j--;
}
}
return true;
}
}
/**
* Definition for a binary tree node.
* public 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;
* }
* }
*/
//官方递归解法
class Solution {
public boolean isSymmetric(TreeNode root) {
return check(root.left, root.right);
}
public boolean check(TreeNode p, TreeNode q) {
//两个都为null返回true
if (p == null && q == null) {
return true;
}
//只有一个为null返回false
if (p == null || q == null) {
return false;
}
return p.val == q.val && check(p.left, q.right) && check(p.right, q.left);
}
}
/**
* Definition for a binary tree node.
* public 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;
* }
* }
*/
//官方迭代解法 迭代解法效率不如递归
class Solution {
public boolean isSymmetric(TreeNode root) {
return check(root, root);
}
public boolean check(TreeNode u, TreeNode v) {
Queue<TreeNode> q = new LinkedList<TreeNode>();
q.offer(u);
q.offer(v);
while (!q.isEmpty()) {
u = q.poll();
v = q.poll();
if (u == null && v == null) {
continue;
}
if ((u == null || v == null) || (u.val != v.val)) {
return false;
}
q.offer(u.left);
q.offer(v.right);
q.offer(u.right);
q.offer(v.left);
}
return true;
}
}