102. Binary Tree Level Order Traversal

Problem:

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,null,null,15,7],

    3
   / \
  9  20
    /  \
   5   7

return its level order traversal as:

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

思路：

Solution (C++):

public:
    vector<vector<int>> levelOrder(TreeNode* root) {
        vector<vector<int>> res;
        levelOrder(root, res, 0);
        return res;
    }
private:
    void levelOrder(TreeNode* root, vector<vector<int>> &res, int depth) {
        if (!root)  return;
        if (res.size() == depth)  res.push_back(vector<int>());

        res[depth].push_back(root->val);
        levelOrder(root->left, res, depth+1);
        levelOrder(root->right, res, depth+1);
    }

性能：

Runtime: 8 ms  Memory Usage: 14.8 MB