Symmetric Tree

Symmetric Tree

问题：

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

思路：

　　dfs

我的代码：

public class Solution {

    public boolean isSymmetric(TreeNode root) {

        if(root == null)    return true;

        return helper(root.left,root.right);

    }

    public boolean helper(TreeNode left, TreeNode right)

    {

        if(left == null && right == null)   return true;

        if(left == null || right == null)   return false;

        return left.val == right.val && helper(left.left, right.right) && helper(left.right, right.left);

    }

}

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