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). http://www.leetcode.com/onlinejudge

leetcode_binary_tree_zigzag_level_order_traversal.cpp

#include <algorithm>
using namespace std;

/**
 * Definition for binary tree
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 * };
 */
class Solution {
public:
    void dfs(vector< vector<int> >& ret, int& maxDepth, int curDepth, TreeNode* root) {
        if (root == NULL) {
            return;
        }
        if (curDepth > maxDepth) {
            maxDepth = curDepth;
            ret.resize(curDepth + 1);
        }
        ret[curDepth].push_back(root->val);
        dfs(ret, maxDepth, curDepth + 1, root->left);
        dfs(ret, maxDepth, curDepth + 1, root->right);
    }

    vector<vector<int> > zigzagLevelOrder(TreeNode *root) {
        vector< vector<int> > ret;
        int maxDepth = -1;
        dfs(ret, maxDepth, 0, root);
        for (int i = 1; i <= maxDepth; i += 2) {
            reverse(ret[i].begin(), ret[i].end());
        }
        return ret;
    }
};