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Leetcode101-symmetricTree

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Solution Report of LeetCode Acceptted

Description

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:

1
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

Solution

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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) {
        if (root == null) return true;
        if (root.left == null && root.right == null) return true;
        return helper(root.left, root.right);
    }
    private boolean helper(TreeNode left, TreeNode right){
        if (left == null && right == null) return true;
        if (left == null || right == null) return false;
        if (left.val != right.val) return false;
        return helper(left.left, right.right) && helper(left.right, right.left);
    }
}