Binary Tree     Breadth-First Search     Depth-First Search     Java     Medium     Tree    

Problem Statement:

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

Solution:

This problem can be solved by BFS.

 
public int maxLevelSum(TreeNode root) 
{
    int curLevel=1, resLevel=0, maxSum=Integer.MIN_VALUE;
    Queue<TreeNode> Q = new LinkedList<>();
    Q.add(root);
    while(!Q.isEmpty())
    {
        int curSum = 0;
        for(int i=Q.size(); i>0; i--)
        {
            TreeNode cur = Q.poll();
            curSum += cur.val;
            if(cur.left!=null) Q.add(cur.left);
            if(cur.right!=null) Q.add(cur.right);
        }
        if (curSum>maxSum)
        {
            maxSum = curSum;
            resLevel = curLevel;
        }
        curLevel++;
    }
    return resLevel;
}

TC: $O(n)$, SC: $O(1)$