binarytree_levelorder.java

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode() {}
 *     TreeNode(int val) { this.val = val; }
 *     TreeNode(int val, TreeNode left, TreeNode right) {
 *         this.val = val;
 *         this.left = left;
 *         this.right = right;
 *     }
 * }
 */
class Solution {
    public List<List<Integer>> levelOrder(TreeNode root) {
        List<List<Integer>> ans = new LinkedList<>();
        Queue<TreeNode> q = new LinkedList<>();
        if(root==null) return ans;
        q.offer(root);
        
        while(!q.isEmpty()){
            int level = q.size();
            List<Integer> list = new LinkedList<>();
            
            for(int i =0; i<level ;i++){
                
                if(q.peek().left!=null) q.offer(q.peek().left);
                if(q.peek().right!=null) q.offer(q.peek().right);
                
                list.add(q.poll().val);
            }
            ans.add(list);
            
        }
        return ans;
    }
}