Trapping Rainwater

#include <bits/stdc++.h>
typedef long long ll;
typedef unsigned long long ull;
typedef long double ld;
#define vi vector<int>
#define vll vector<long long>
#define pb push_back
#define vpll vector<pair<ll, ll>>
#define vpi vector<pair<int, int>>
#define p_b pop_back
#define FOR(a, b) for(int i = a; i <= b; i++)
#define tests() int t; cin >> t; while(t--)
#define input(a, n) for(int i = 0; i < n; i++){int x; cin >> x; a.pb(x);}
#define MOD (ull)1000000007
using namespace std;

ll trappingWater(int arr[], int n){
    int i = 0, k = 0;
    ll sum = 0;
    bool f = true;
    while(i < n){
        int j = i;
        //check for values in the array after i which are less than or equal to arr[i]
        while(arr[j] <= arr[i] && j < n){
            j++;
        }
        /*if the loop reaches the end of the array then it means that the last element of the array
        is less than arr[i], hence the we should calculate in the reverse order upto i to find the sum,
        if its not the case then simply calculate the differences with arr[i] and we will get the total volume*/
        if(j == n){
            f = false;
            k = i;
            break;
        } else{
            int tmp = j;
            j--; //decremented due to fact that arr[j] is greater than arr[i]
            while(j >= i){
                sum += arr[i]-arr[j];
                j--;
            }
            i = tmp;
        }
    }
    if(!f){
        /*if the first case is satisfied in the earlier loop then find the differences from the back of the array*/
        i = n-1;
        while(i >= k){
            int j = i;
            while(arr[j] <= arr[i] && j >= k){
                sum += arr[i]-arr[j];
                j--;
            }
            i = j;
        }
    }
    return sum;
}

int main(){
    ios_base::sync_with_stdio(false);
    cin.tie(NULL);
    int n;
    cin >> n;
    int arr[n];
    for(int i = 0; i < n; i++){
        cin >> arr[i];
    }
    cout << trappingWater(arr, n);
    return 0;
}