给定一个二叉树，检查它是否是镜像对称的。

例如，二叉树 [1,2,2,3,4,4,3] 是对称的。

此题与上一题有异曲同工之妙，[LeetCode] #100 相同的树

思路基本一样，递归解法

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode() {}
 *     TreeNode(int val) { this.val = val; }
 *     TreeNode(int val, TreeNode left, TreeNode right) {
 *         this.val = val;
 *         this.left = left;
 *         this.right = right;
 *     }
 * }
 */
class Solution {
    public boolean isSymmetric(TreeNode root) {
        return check(root, root);
    }

    public boolean check(TreeNode p, TreeNode q) {
        if (p == null && q == null) return true;
        if (p == null || q == null) return false;
        return p.val == q.val && check(p.left, q.right) && check(p.right, q.left);
    }
}

非递归解法

class Solution {
    public boolean isSymmetric(TreeNode root) {
        TreeNode l = root.left,r = root.right;
        Queue<TreeNode> tmpQueue = new LinkedList<TreeNode>();
        tmpQueue.offer(l);
        tmpQueue.offer(r);
        while(!tmpQueue.isEmpty()){
            l = tmpQueue.poll();
            r = tmpQueue.poll();
            if(l == null && r == null){
                continue;
            }
            if((l == null || r == null) || l.val != r.val){
                return false;
            }
            tmpQueue.offer(l.left);
            tmpQueue.offer(r.right);

            tmpQueue.offer(l.right);
            tmpQueue.offer(r.left);
        }
        return true;
    }
}

知识点：无

总结：无