q102_二叉树的层次遍历

package 树的遍历.q102_二叉树的层次遍历;
public class TreeNode {
    int val;
    TreeNode left;
    TreeNode right;
    TreeNode(int x) {
        val = x;
    }
}

package 树的遍历.q102_二叉树的层次遍历;
import java.util.ArrayList;
import java.util.LinkedList;
import java.util.List;
import java.util.Queue;
/**
 * 利用队列迭代 o(n)
 */
public class Solution {
    public List<List<Integer>> levelOrder(TreeNode root) {
        List<List<Integer>> levels = new ArrayList<>();
        if (root == null) {
            return levels;
        }
        Queue<TreeNode> queue = new LinkedList<>();
        queue.add(root);
        int level = 0;
        while (!queue.isEmpty()) {
            levels.add(new ArrayList<>());
            int levelLength = queue.size();
            for (int i = 0; i < levelLength; ++i) {
                TreeNode node = queue.remove();
                levels.get(level).add(node.val);
                if (node.left != null) {
                    queue.add(node.left);
                }
                if (node.right != null) {
                    queue.add(node.right);
                }
            }
            level++;
        }
        return levels;
    }
}