Leetcode: Binary Tree Zigzag Level Order Traversal

Given a binary tree, return the zigzag level order traversal of its nodes' values. (ie, from left to right, then right to left for the next level and alternate between).

For example:
Given binary tree {3,9,20,#,#,15,7},

    3   / \  9  20    /  \   15   7

return its zigzag level order traversal as:

[  [3],  [20,9],  [15,7]]

用stack, 从左到右，从右到左，轮着先入栈就行。

/** * Definition for binary tree * struct TreeNode { *     int val; *     TreeNode *left; *     TreeNode *right; *     TreeNode(int x) : val(x), left(NULL), right(NULL) {} * }; */class Solution {public:    vector<vector<int> > zigzagLevelOrder(TreeNode *root) {        vector<vector<int> > result;        if (root == NULL) {            return result;        }                bool from_left = true;        TreeNode *cur;        stack<TreeNode*> nodes, tmp;        nodes.push(root);                do {            vector<int> level;            while (!nodes.empty()) {                cur = nodes.top();                nodes.pop();                level.push_back(cur->val);                if (from_left) {                    if (cur->left != NULL) {                        tmp.push(cur->left);                    }                    if (cur->right != NULL) {                        tmp.push(cur->right);                    }                }                else {                    if (cur->right != NULL) {                        tmp.push(cur->right);                    }                    if (cur->left != NULL) {                        tmp.push(cur->left);                    }                }            }            result.push_back(level);            swap(nodes, tmp);            from_left = !from_left;         } while (!nodes.empty());                  return result;    }};

=====================第二次========================

/** * Definition for binary tree * struct TreeNode { *     int val; *     TreeNode *left; *     TreeNode *right; *     TreeNode(int x) : val(x), left(NULL), right(NULL) {} * }; */class Solution {public:    vector<vector<int> > zigzagLevelOrder(TreeNode *root) {        vector<vector<int>> result;        bool from_left = true;        stack<TreeNode*> first;        stack<TreeNode*> second;        first.push(root);                vector<int> level;        TreeNode *cur = NULL;        while (!first.empty()) {            while (!first.empty()) {                cur = first.top();                first.pop();                if (cur != NULL) {                    level.push_back(cur->val);                    if (from_left) {                        second.push(cur->left);                        second.push(cur->right);                    }                    else {                        second.push(cur->right);                        second.push(cur->left);                    }                }            }                        if (!level.empty()) {                result.push_back(level);                level.clear();            }            first.swap(second);            from_left = !from_left;        }                return result;    }};