102

102. Binary Tree Level Order Traversal (Medium)

Given the root of a binary tree, return the level order traversal of its nodes' values. (i.e., from left to right, level by level).

 

Example 1:

Input: root = [3,9,20,null,null,15,7]
Output: [[3],[9,20],[15,7]]

Example 2:

Input: root = [1]
Output: [[1]]

Example 3:

Input: root = []
Output: []

 

Constraints:

• The number of nodes in the tree is in the range [0, 2000].
• -1000 <= Node.val <= 1000

Related Topics:
Tree, Breadth-First Search, Binary Tree

Similar Questions:

• Binary Tree Zigzag Level Order Traversal (Medium)
• Binary Tree Level Order Traversal II (Medium)
• Minimum Depth of Binary Tree (Easy)
• Binary Tree Vertical Order Traversal (Medium)
• Average of Levels in Binary Tree (Easy)
• N-ary Tree Level Order Traversal (Medium)
• Cousins in Binary Tree (Easy)

Solution 1.

// OJ: https://leetcode.com/problems/binary-tree-level-order-traversal/
// Author: github.com/lzl124631x
// Time: O(N)
// Space: O(N)
class Solution {
public:
    vector<vector<int>> levelOrder(TreeNode* root) {
        if (!root) return {};
        vector<vector<int>> ans;
        queue<TreeNode*> q{{root}};
        while (q.size()) {
            ans.emplace_back();
            for (int cnt = q.size(); cnt--; ) {
                auto n = q.front();
                q.pop();
                ans.back().push_back(n->val);
                if (n->left) q.push(n->left);
                if (n->right) q.push(n->right);
            }
        }
        return ans;
    }
};