Problem 102: traversal of binary tree sequence (breadth first search)

1. Title

2. My initial thoughts and problems

Slightly, look directly at the solution

3. Problem solving method 1: breadth first search

class Solution {
    
    
    public List<List<Integer>> levelOrder(TreeNode root) {
    
    
        List<List<Integer>> ret = new ArrayList<List<Integer>>();
        if (root == null) {
    
    
            return ret;
        }

        Queue<TreeNode> queue = new LinkedList<TreeNode>();
        queue.offer(root);
        while (!queue.isEmpty()) {
    
    
            List<Integer> level = new ArrayList<Integer>();
            int currentLevelSize = queue.size();
            for (int i = 1; i <= currentLevelSize; ++i) {
    
    
                TreeNode node = queue.poll();
                level.add(node.val);
                if (node.left != null) {
    
    
                    queue.offer(node.left);
                }
                if (node.right != null) {
    
    
                    queue.offer(node.right);
                }
            }
            ret.add(level);
        }
        
        return ret;
    }
}