LeetCode: Binary Tree Zigzag Level Order Traversal

思路：和前面一题的层次遍历有点区别，这里需要用一个栈结构来保存每一层的节点，因为下一层遍历始于当前层的最后一个节点，而且奇数层是从左至右开始遍历，偶数层是从右向左遍历。

code:

class Solution {
public:
    vector<vector<int> > zigzagLevelOrder(TreeNode *root) {
        vector<vector<int> > ret;
        if(root){
            stack<TreeNode *> preQ, nextQ;
            TreeNode * p = root;
            int flag = 0;
            preQ.push(p);
            while(!preQ.empty()){
                vector<int> temp;
                if(flag == 0){
                    while(!preQ.empty()){
                        TreeNode *t = preQ.top();
                        temp.push_back(t->val);
                        if(t->left != NULL)
                            nextQ.push(t->left);
                        if(t->right != NULL)
                            nextQ.push(t->right);
                        preQ.pop();
                    }
                    flag = 1;
                }
                else{
                     while(!preQ.empty()){
                        TreeNode *t = preQ.top();
                        temp.push_back(t->val);
                        if(t->right != NULL)
                            nextQ.push(t->right);
                        if(t->left != NULL)
                            nextQ.push(t->left);
                        preQ.pop();
                    }
                    flag = 0;
                }
                ret.push_back(temp);
                preQ = nextQ;
                while(!nextQ.empty())
                    nextQ.pop();
            }
        }
        return ret;
    }
};