[LeetCode]101. Symmetric Tree

题目描述: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   2
/ \ / \
3  4 4  3

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

1
/ \
2   2
\   \
3    3

Note:

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

解题思路：判断一棵二叉树是否为镜像二叉树，只要判断左子树的节点值是否等于右子树节点值即可。与判断两个二叉树的方法类似。

public boolean isSymmetric(TreeNode root) {
if(root==null)return true;
return isSameTree(root.left, root.right);
}

public boolean isSameTree(TreeNode p,TreeNode q){
//镜像判断是否相等
if(p==null&&q==null)return true;
if(p==null||q==null)return false;
if(p.val==q.val){
return isSameTree(p.left,q.right)&&isSameTree(p.right,q.left);
}else{
return false;
}
}