LeetCode102 Binary Tree Level Order Traversal 二叉树分层遍历

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
    /  \
   15   7

return its level order traversal as:

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

题源：here；完整实现：here

思路：

我们每一次递归都会向下一层移动。我们使用一个变量来记录到了第几层来。代码如下：

void helper(TreeNode *root, int layer, vector<vector<int>> &solutions){
	if (!root) return;
	layer++;
	if (solutions.size() < layer) solutions.push_back({});
	solutions[layer - 1].push_back(root->val);
	if (root->left) helper(root->left, layer, solutions);
	if (root->right) helper(root->right, layer, solutions);
}
vector<vector<int>> levelOrder(TreeNode* root) {
	vector<vector<int>> solutions;
	helper(root, 0, solutions);
	return solutions;
}