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

/*
 * @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) {
        Deque<TreeNode> deq = new ArrayDeque<>();
        List<List<Integer>> res = new ArrayList<>();
        if (root == null) {
            return res;
        }
        deq.offerLast(root);
        while (!deq.isEmpty()) {
            int size = deq.size();
            List<Integer> list = new ArrayList<>();
            for (int i = 0; i < size; i++) {
                TreeNode cur = deq.pollFirst();
                list.add(cur.val);
                if (cur.left!= null) {
                    deq.offerLast(cur.left);
                }
                if (cur.right!= null) {
                    deq.offerLast(cur.right);
                }
            }
            res.add(list);
        }
        return res;
    }
}
// @lc code=end