【Leetcode長征系列】Symmetric Tree

原題：

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

思路：遍歷每一層的數據，空節點用特殊字符代替，判斷每一層是否是對稱。

代碼：

/**
 * Definition for binary tree
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 * };
 */
class Solution {
public:
    bool isSymmetric(TreeNode *root) {
        if(root==NULL || (root->right==NULL && root->left==NULL)) return true;
        queue<TreeNode*> cur, next;
        deque<int> row;
        TreeNode *tmp;
        
        cur.push(root);
        while(!cur.empty()){
            tmp = cur.front();
            cur.pop();
            if(tmp->left!=NULL) {
                next.push(tmp->left);
                row.push_back(tmp->left->val);
            }
            else row.push_back(INT_MIN);
            if(tmp->right!=NULL) {
                next.push(tmp->right);
                row.push_back(tmp->right->val);
            }
            else row.push_back(INT_MIN);
            if(cur.empty()) {
                if(!isSym(row)) return false;
                cur = next;
                while(!next.empty()){
                    next.pop();
                }
            }
        }
        return true;
    }
    
    bool isSym(deque<int> &row){
        while(!row.empty()){
            if(row[0]!=row[row.size()-1]) return false;
            row.pop_back();
            row.pop_front();
        }
        return true;
    }
};