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SymmetricTree

Problem101

Given a binary tree, check whether it is a mirror of itself (ie, symmetric around its center).

For example, this binary tree [1,2,2,3,4,4,3] is symmetric:

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2
3
4
5
    1
/ \
2 2
/ \ / \
3 4 4 3

But the following [1,2,2,null,3,null,3] is not:

1
2
3
4
5
  1
/ \
2 2
\ \
3 3

Note:
Bonus points if you could solve it both recursively and iteratively.

key

一道验证树是否是对称的问题,主要采取递归的方法

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/**
* Definition for a binary tree node.
* public class TreeNode {
* int val;
* TreeNode left;
* TreeNode right;
* TreeNode(int x) { val = x; }
* }
*/
class Solution {
public boolean isSymmetric(TreeNode root) {
return isMirror(root,root);
}
public boolean isMirror(TreeNode root,TreeNode self){
if(root==null && self==null)return true;
if(root==null ||self==null) return false;
return root.val==self.val && isMirror(root.left,self.right)&&isMirror(root.right,self.left);

}
}