lc102-Binary Tree Level Order Traversal

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/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 * };
 */
 // recursive ver.
class Solution {
    vector<vector<int>> res;
public:
    void traverse(TreeNode* r, int level) {
        if (level > res.size()) res.push_back(vector<int>());
        res[level - 1].push_back(r -> val);
        if (r -> left != nullptr) traverse(r -> left, level+1);
        if (r -> right != nullptr) traverse(r -> right, level+1);
    }
    vector<vector<int>> levelOrder(TreeNode* root) {
        if (root != nullptr) traverse(root, 1);
        return res;
    }
};