101. Symmetric Tree

题目：

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

For example, this binary tree is symmetric:

    1

   / \

  2   2

 / \ / \

3  4 4  3

 

But the following is not:

    1

   / \

  2   2

   \   \

   3    3

 

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

confused what "{1,#,2,3}" means? > read more on how binary tree is serialized on OJ.

 

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  Tree Depth-first Search

 

链接：http://leetcode.com/submissions/detail/11328058/

题解：

DFS。

Time Complexity - O(n)， Space Complexity - O(1)。

public class Solution {

    public boolean isSymmetric(TreeNode root) {

        if(root == null)

            return true;

        else

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

    }

    

    private boolean isSymmetric(TreeNode left, TreeNode right){

        if(left == null && right == null)

            return true;

        if(left != null && right != null && left.val == right.val)

            return isSymmetric(left.left, right.right) && isSymmetric(left.right, right.left);

        else

            return false;

    }

}

 

测试：