LeetCode 1161. Maximum Level Sum of a Binary Tree

原题链接在这里：https://leetcode.com/problems/maximum-level-sum-of-a-binary-tree/description/

题目：

Given the root of a binary tree, the level of its root is 1, the level of its children is 2, and so on.

Return the smallest level x such that the sum of all the values of nodes at level x is maximal.

Example 1:

Input: root = [1,7,0,7,-8,null,null]
Output: 2
Explanation: 
Level 1 sum = 1.
Level 2 sum = 7 + 0 = 7.
Level 3 sum = 7 + -8 = -1.
So we return the level with the maximum sum which is level 2.

Example 2:

Input: root = [989,null,10250,98693,-89388,null,null,null,-32127]
Output: 2

Constraints:

• The number of nodes in the tree is in the range [1, 104].
• -105 <= Node.val <= 105

题解：

Perforom level order traversal. And accumlate sum of each level.

When there is a new maximum, update the max and result level.

Time Complexity: O(n). n is number of nodes in the tree.

Space: O(n).

AC Java:

/**
 * 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 int maxLevelSum(TreeNode root) {
        if(root == null){
            return 0;
        }

        int max = Integer.MIN_VALUE;
        int res = 0;
        int level = 1;
        LinkedList<TreeNode> que = new LinkedList<>();
        que.add(root);
        while(!que.isEmpty()){
            int size = que.size();
            int sum = 0;
            while(size-- > 0){
                TreeNode cur = que.poll();
                sum += cur.val;
                if(cur.left != null){
                    que.add(cur.left);
                }

                if(cur.right != null){
                    que.add(cur.right);
                }
            }

            if(sum > max){
                max = sum;
                res = level;
            }

            level++;
        }

        return res;
    }
}