leetcode_101. Symmetric Tree

題目：

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.

分析：

左右對稱，左子數與右子樹又對稱，左的左 和右的右又相同，所以用兩個queue來順序分別存 左的左 和 右的右。對比即可。

代碼：

/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 * };
 */
class Solution {
public:
    bool isSymmetric(TreeNode* root) {
        TreeNode *l, *r;
        queue<TreeNode*> p,q;
        if(root == NULL){
            return true;
        }
        if(root->left == NULL && root->right == NULL){
            return true;
        }
        if(root->left == NULL || root->right == NULL){
            return false;
        }
        p.push(root->left);
        q.push(root->right);
        
        
        while(!q.empty() && !p.empty()){
            l = p.front();
            r = q.front();
            p.pop();
            q.pop();
            if(l == NULL && r == NULL){
                continue;
            }
            if(l == NULL || r == NULL){
                return false;
            }
            if(l->val != r->val){
                return false;
            }
           
            p.push(l->left);
            p.push(l->right);
            q.push(r->right);
            q.push(r->left);
        }
        return true;
    }
};