【Leetcode】Symmetric Tree

题目链接：https://leetcode.com/problems/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.

思路：

注意我在递归中保存结果的方式，跟Kth Smallest Element in a BST 算法2和Balanced Binary
Tree 中是一样的。

基本框架是：

boolean result = true;
public boolean recur(TreeNode root){
if(root!=null){
if(satisfy a condition){
result = false;
}
recur(root.left);
recur(root.right);
}
return result;
}

算法：

public boolean isSym(TreeNode p, TreeNode q) {
if (p == null && q == null) {//都为空
return true;
}else if (p == null || q == null) {//有一个不为空
result = false;
}else if (p.val == q.val) {
isSym(p.left, q.right);//注意对称
isSym(p.right, q.left);
} else {
result = false;
}
return result;
}

public boolean isSymmetric(TreeNode root) {
if (root == null) {
return true;
}
if (root.left == null && root.right == null) { //单结点
return true;
}
if (root.left == null || root.right == null) {//不对称
return false;
}
return isSym(root.left, root.right);
}

boolean result = true;