binary tree zigzag level order traversal.

binary_tree_zigzag_level_order_traversal.cpp

/**
 * 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) {
      //push BFS to two queues; once current queue empty, switch
      //    note to reverse the vector every other level
        vector<vector<int> > vvInt;
        queue<TreeNode *> parentQ;  
        if (root)
            parentQ.push(root);
        queue<TreeNode *> childrenQ;
        bool forwardDirection = true;
        vector<int> vInt;
        while ( !parentQ.empty() ) {
            TreeNode * aNode = parentQ.front();
            parentQ.pop();
            vInt.push_back(aNode->val);
            if (aNode->left)
                childrenQ.push(aNode->left);
            if (aNode->right)
                childrenQ.push(aNode->right);
            if (parentQ.empty()) {
                //one level is done
                //switch to next level
                swap(parentQ, childrenQ);
                if (!forwardDirection) //reverse vInt
                    reverse(vInt.begin(), vInt.end());
                
                vvInt.push_back(vInt);
                vInt.clear();
                forwardDirection = !forwardDirection;
            }
        }
        return vvInt;
        
    }
};