My solution without using a global variable.

class Solution {
    private class Int {
        int val;
        Int(int val) {
            this.val = val;
        }
    }
    public int diameterOfBinaryTree(TreeNode root) {
        if (root == null) return 0;
        Int ans = new Int(0);
        depthOfTree(root, ans);
        return ans.val;
    }
    
    public int depthOfTree(TreeNode root, Int ans) {
        if (root == null) return 0;
        int l = depthOfTree(root.left, ans);
        int r = depthOfTree(root.right, ans);
        ans.val = Math.max(ans.val, l + r);
        return 1 + Math.max(l, r);
    }
}

import random
class Solution(object):
    def findKthLargest(self, nums, k):
        """
        :type nums: List[int]
        :type k: int
        :rtype: int
        """
        return self.recursive(nums, 0, len(nums) - 1, k)

    def recursive(self, nums, start, end, k):
        if start == end and k == 1:
            return nums[start]
        i = random.randrange(start, end + 1)
        nums[end], nums[i] = nums[i], nums[end]
        j = start
        for i in range(start, end):
            if nums[i] >= nums[end]:
                nums[i], nums[j] = nums[j], nums[i]
                j += 1
        nums[j], nums[end] = nums[end], nums[j]
        if k < (j - start + 1):
            return self.recursive(nums, start, j-1, k)
        elif k > (j - start + 1):
            return self.recursive(nums, j+1, end, k - (j - start + 1))
        else:
            return nums[j]
                
            
        
        
        
        
        

int maxProfit(vector<int>& prices) {
    int days = prices.size();
    if (days <= 1) return 0;
    int profit = 0;
    for (int i = 1; i < days; i++) {
        if (prices[i] > prices[i - 1]) {
            profit += prices[i] - prices[i - 1];
        }
    }
    return profit;
}