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

/*
 * @lc app=leetcode id=102 lang=java
 *
 * [102] Binary Tree Level Order Traversal
 *
 * https://leetcode.com/problems/binary-tree-level-order-traversal/description/
 *
 * algorithms
 * Medium (47.06%)
 * Total Accepted:    352.5K
 * Total Submissions: 740.6K
 * Testcase Example:  '[3,9,20,null,null,15,7]'
 *
 * 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],
 *
 * ⁠   3
 * ⁠  / \
 * ⁠ 9  20
 * ⁠   /  \
 * ⁠  15   7
 *
 *
 *
 * return its level order traversal as:
 *
 * [
 * ⁠ [3],
 * ⁠ [9,20],
 * ⁠ [15,7]
 * ]
 *
 *
 */
/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
class Solution {
  public List<List<Integer>> levelOrder(TreeNode root) {
    List<List<Integer>> result = new ArrayList<>();
    if (root == null) {
      return result;
    }

    Queue<TreeNode> queue = new LinkedList<>();
    queue.offer(root);

    while (!queue.isEmpty()) {
      List<Integer> level = new ArrayList<>();
      int size = queue.size();
      for (int i = 0; i < size; 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);
        }
      }
      result.add(level);
    }

    return result;
  }
}