102. 二叉树的层序遍历（高频题）

102. 二叉树的层序遍历

• 解题思路
• 代码

解题思路

没啥说的，题目都告诉你要用层级遍历了，也就是BFS。

代码

/**
 * 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) {
        if (root == null) {
            return new ArrayList<>();
        }

        Queue<TreeNode> q = new LinkedList<>();
        q.add(root);
        List<List<Integer>> res = new ArrayList<>();

        while (!q.isEmpty()) {
            int size = q.size();
            List<Integer> temp = new ArrayList<>();
            for (int i = 0; i < size; i++) {
                TreeNode cur = q.poll();
                temp.add(cur.val);

                if (cur.left != null) {
                    q.add(cur.left);
                }
                if (cur.right != null) {
                    q.add(cur.right);
                }
            }
            res.add(temp);

        }
        return res;
    }
}