LeetCode(101):Symmetric Tree

Symmetric Tree: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.

题意：判断二叉树是否对称。

思路：使用递归的方法同时对两棵子树进行递归判断。

代码：

public class Solution {
    public boolean isSymmetric(TreeNode root) {
        if(root==null) return true;
        return ifSymmetric(root.left,root.right);
    }
    public boolean ifSymmetric(TreeNode tree1,TreeNode tree2){
        if(tree1==null&&tree2==null)
            return true;
        else if(tree1==null || tree2==null)
            return false;
        if(tree1.val !=tree2.val)
            return false;
        else
        return (ifSymmetric(tree1.left,tree2.right) && ifSymmetric(tree1.right,tree2.left));
    }
}