102-levelOrder.java

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
import java.util.*;

class TreeNode {
    int val;
    TreeNode left;
    TreeNode right;
    TreeNode(int x) { val = x; }
}
class Solution_102 {
    public List<List<Integer>> levelOrder(TreeNode root) {
        List<List<Integer>> res = new ArrayList<List<Integer>>();
        if(root == null) return res;
        Queue<TreeNode> queue = new LinkedList<TreeNode>();
        queue.add(root);
        while(queue.size() != 0){
            int count = queue.size();
            List<Integer> l = new ArrayList<Integer>();
            while(count > 0){
                TreeNode cur = queue.poll();
                
                l.add(cur.val);
                if(cur.left!=null){
                    queue.add(cur.left);
                }
                if(cur.right!=null){
                    queue.add(cur.right);
                }
                count--;
            }
            res.add(l);
        }
        return res;
    }
    
}