Problem: Binary Tree Level Order Traversal

Question

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],

return its level order traversal as:

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

Solution

Breadth First Search (BFS) can be used to solve the problem.

Code

Here is a sample solution.

public class Solution {
    public List<List<Integer>> levelOrder(TreeNode root) {
        List<List<Integer>> res = new ArrayList<>();
        if(root == null) return res;
        
        Queue<TreeNode> q = new LinkedList<>();
        q.add(root);
        
        while(!q.isEmpty()) {
            
            int curLevelCount = q.size();
            List<Integer> layerValues = new ArrayList<>();
            
            while(curLevelCount-- > 0) {
                TreeNode node = q.poll();
                layerValues.add(node.val);
                
                if(node.left != null) 
                    q.add(node.left);
                if(node.right != null) 
                    q.add(node.right);
            }
            
            res.add(layerValues);
        }
        
        return res;
    }
}

Performance

Each node is pushed and polled from the queue once, so the overall time complexity is O(n).

Problem: Binary Tree Level Order Traversal

Question

Solution

Code

Performance

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