102. Binary Tree Level Order Traversal

102. Binary Tree Level Order Traversal (Medium)

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]
]

Related Topics:
Tree, Breadth-first Search

Similar Questions:

• Binary Tree Zigzag Level Order Traversal (Medium)
• Binary Tree Level Order Traversal II (Easy)
• 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;
        q.push(root);
        while (q.size()) {
            int cnt = q.size();
            ans.emplace_back();
            while (cnt--) {
                root = q.front();
                q.pop();
                ans.back().push_back(root->val);
                if (root->left) q.push(root->left);
                if (root->right) q.push(root->right);
            }
        }
        return ans;
    }
};