LeetCode-102-二叉树的层序遍历

LeetCode-102-二叉树的层序遍历

102. 二叉树的层序遍历

难度中等889收藏分享切换为英文接收动态反馈

给你一个二叉树，请你返回其按 层序遍历 得到的节点值。 （即逐层地，从左到右访问所有节点）。

示例：
二叉树：[3,9,20,null,null,15,7],

    3
   / \
  9  20
    /  \
   15   7

返回其层序遍历结果：

[
  [3],
  [9,20],
  [15,7]
]

---

思路

二叉树的基本算法

image-20210526163617201

image-20210526163837652

image-20210526163940564

image-20210526164054104

image-20210526164139989

class Solution {
    public static List> levelOrder(TreeNode root) {
        List> ans = new ArrayList<>();
        if (root == null) {
            return ans;
        }
        LinkedList deque = new LinkedList<>();
        deque.add(root);
        int size = 0;
        while(!deque.isEmpty()) {
            size = deque.size();
            List curLevel = new ArrayList<>();
            for(int i = 0 ; i< size;i++) {
                TreeNode cur = deque.pollLast();
                curLevel.add(cur.val);
                if(cur.left != null) {
                    deque.addFirst(cur.left);
                }
                if(cur.right != null) {
                    deque.addFirst(cur.right);
                }
            }
            ans.add(curLevel);
        }
        return ans;
    }

}

image-20210605152331200

递归解法

class Solution {
   List> res = new ArrayList<>();
    
    public List> levelOrder(TreeNode root) {
        if(root == null) return res;
        return helper(root, 0);
    }

    private List> helper (TreeNode root, int level) {
        if(root != null){
            if(res.size() <= level){
                List cur = new ArrayList<>();
                res.add(cur);
            }
            res.get(level).add(root.val);
            helper(root.left, level+1);
            helper(root.right, level+1);
        }
        return res;
    }

}

image-20210605152519194