1- 思路
借助队列
- 1- 创建队列：
Queue<TreeNode> queue，初始化加入 root.left 和 root.right - 2- 判断逻辑：while(!queue.isEmpty())
- 2-1 弹出两个结点： 分别为
l 和 r
- ① 情况1：
l == null 且 r ==null 此时，直接 continue - ② 情况2：
l == null 且 r !=null 此时，直接 返回 false - ③ 情况3：
l != null 且 r ==null 此时，直接 返回 false - ④ 情况4：
l.val != r.val 直接返回 false
- 2-2 入栈顺序
- 先入栈
l.left ，再入栈 r.right - 先入栈
l.right，再入栈 r.left
2- 实现
⭐101. 对称二叉树——题解思路
class Solution {
public boolean isSymmetric(TreeNode root) {
Queue<TreeNode> queue = new LinkedList<>();
queue.offer(root.left);
queue.offer(root.right);
while(!queue.isEmpty()){
TreeNode l = queue.poll();
TreeNode r = queue.poll();
if( l == null && r==null){
continue;
}
if( l== null && r!=null ){
return false;
}
if(l!=null && r == null){
return false;
}
if(l.val!=r.val){
return false;
}
queue.offer(l.left);
queue.offer(r.right);
queue.offer(l.right);
queue.offer(r.left);
}
return true;
}
}
3- ACM 实现
public class isSymmetric {
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;
}
}
public static TreeNode build(String str){
if(str == null || str.length()==0){
return null;
}
String input = str.replace("[","");
input = input.replace("]","");
String[] parts = input.split(",");
Integer[] nums = new Integer[parts.length];
for(int i = 0 ; i < parts.length;i++){
if(!parts[i].equals("null")){
nums[i] = Integer.parseInt(parts[i]);
}else{
nums[i] = null;
}
}
Queue<TreeNode> queue = new LinkedList<>();
TreeNode root = new TreeNode(nums[0]);
queue.offer(root);
int index = 1;
while(!queue.isEmpty()&& index<parts.length){
TreeNode node = queue.poll();
if(index<nums.length && nums[index]!=null){
node.left = new TreeNode(nums[index]);
queue.offer(node.left);
}
index++;
if(index<nums.length && nums[index]!=null){
node.right = new TreeNode(nums[index]);
queue.offer(node.right);
}
index++;
}
return root;
}
public static boolean isSymetric(TreeNode root){
Queue<TreeNode> queue = new LinkedList<>();
queue.offer(root.left);
queue.offer(root.right);
while(!queue.isEmpty()){
TreeNode l = queue.poll();
TreeNode r = queue.poll();
if(l==null && r==null){
continue;
}
if(l!=null && r==null){
return false;
}
if(l==null && r!=null){
return false;
}
if(l.val!=r.val){
return false;
}
queue.offer(l.left);
queue.offer(r.right);
queue.offer(l.right);
queue.offer(r.left);
}
return true;
}
public static void main(String[] args) {
Scanner sc = new Scanner(System.in);
String input = sc.nextLine();
TreeNode root = build(input);
System.out.println("结果是"+isSymetric(root));
}
}
