Given a binary tree, return the level order traversal of its nodes' values. (ie, from left to right, level by level). For example: Given binary tree {3,9,20,#,#,15,7}, 3 / \ 9 20 / \ 15 7 return its level order traversal as: [ [3], [9,20], [15,7] ]

Binary Tree 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> > levelOrder(TreeNode *root) {
        // Start typing your C/C++ solution below
        // DO NOT write int main() function
        vector<vector<int> > res;
        
        cal(res, 0, root);
        
        return res;
        
    }
    
    
    void cal(vector<vector<int> >& res, int curLevel, TreeNode* node) {
        if(node == NULL) {
            return;
        }
        
        if(res.size() < curLevel + 1) {
            res.push_back(vector<int>());
        }
        
        res[curLevel].push_back(node->val);
        cal(res, curLevel + 1, node->left);
        cal(res, curLevel + 1, node->right);
    }
};