Classic Interview 150 Questions P102 Binary Tree Level Order Traversal

---

Timeline

Timeline

2025-10-31

init

Level order traversal

---

Problem:

P102 Binary tree level order traversal

https://leetcode.cn/problems/binary-tree-level-order-traversal/description/?envType=study-plan-v2&envId=top-interview-150

Level-order traversal

struct TreeNode {
	int val;
	TreeNode *left;
	TreeNode *right;
	TreeNode()
		: val(0)
		, left(nullptr)
		, right(nullptr)
	{
	}
	TreeNode(int x)
		: val(x)
		, left(nullptr)
		, right(nullptr)
	{
	}
	TreeNode(int x, TreeNode *left, TreeNode *right)
		: val(x)
		, left(left)
		, right(right)
	{
	}
};

#include <vector>
#include <queue>
#include <utility>
using std::vector;
using std::queue;

class Solution {
    public:
	vector<vector<int> > levelOrder(TreeNode *root)
	{
		int i, n;
		queue<TreeNode *> que;
		vector<vector<int> > res;
		TreeNode *p;
		res.reserve(32); // Reserve space to avoid multiple reallocations
		if (root == nullptr) {
			return res;
		}
		que.push(root);
		while (!que.empty()) {
			n = que.size();
			vector<int> vec(n);
			for (i = 0; i < n; i++) {
				p = que.front();
				que.pop();
				vec[i] = p->val;

				if (p->left)
					que.push(p->left);

				if (p->right)
					que.push(p->right);

			}
			res.push_back(std::move(vec));
		}
		return res;
	}
};

leetcode hot100 rewrite

#include <vector>
#include <queue>

using std::vector;
using std::queue;

class Solution {
    public:
        vector<vector<int> > levelOrder(TreeNode *root)
        {
                int i, n;
                vector<vector<int> > res;
                queue<TreeNode *> que;
                TreeNode *p;

                if (root)
                        que.push(root);

                while (!que.empty()) {
                        n = que.size();
                        vector<int> curr_level;
                        for (i = 0; i < n; i++) {
                                p = que.front();
                                que.pop();
                                curr_level.push_back(p->val);

                                if (p->left)
                                        que.push(p->left);
                                if (p->right)
                                        que.push(p->right);
                        }
                        res.push_back(curr_level);
                }
                return res;
        }
};