力扣——对称二叉树

对称二叉树

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？

/**
 * 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;
 *     }
 * }
 */
class Solution {
    
    
    public boolean isSymmetric(TreeNode root) {
    
    
        return check(root, root);
    }

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

---

package com.kk;

public class IsSymmetric {
    
    
    public boolean isSymmetric(TreeNode root) {
    
    
        return check(root, root);
    }

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