102.binary-tree-level-order-traversal.java

import java.util.ArrayList;
import java.util.LinkedList;
import java.util.List;
import java.util.PriorityQueue;
import java.util.Queue;

import javax.sound.midi.Soundbank;
import javax.swing.tree.TreeNode;

/*
 * @lc app=leetcode id=102 lang=java
 *
 * [102] Binary Tree Level Order Traversal
 */

// @lc code=start
/**
 * 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>> res = new ArrayList<>();
        Queue<TreeNode> q = new LinkedList<>();

        if (root != null) {
            q.add(root);
        }

        // BFS
        while (!q.isEmpty()) {
            List<Integer> currLevelNodes = new ArrayList<>();
            int currLevelSize = q.size();

            // use for loop to traverse all the elements in current level
            for (int i = 0; i < currLevelSize; i++) {
                TreeNode node = q.poll();
                currLevelNodes.add(node.val);

                if (node.left != null) {
                    q.add(node.left);
                }

                if (node.right != null) {
                    q.add(node.right);
                }
            }
            res.add(currLevelNodes);
        }

        return res;
    }
}
// @lc code=end