Main.java

import java.util.ArrayList;
import java.util.Arrays;
import java.util.List;

public class Main {

    public static void main(String[] args) {

        int[] case1 = {2, 0, 2};
        int[] case2 = {3, 0, 0, 2, 0, 4};
        int[] case3 = {0, 1, 0, 2, 1, 0, 1, 3, 2, 1, 2, 1};
        int[] case4 = {1, 4, 1, 2, 5, 1, 2, 3, 1};
        int[] case5 = {5, 0, 4, 0, 3, 0, 2}; //9 output


        System.out.println("water that's filled will be == " + calculateTheVolumeWaterCanReside(case1, case1.length));
        System.out.println("water that's filled will be == " + calculateTheVolumeWaterCanReside(case2, case2.length));
        System.out.println("water that's filled will be == " + calculateTheVolumeWaterCanReside(case3, case3.length));
        System.out.println("water that's filled will be == " + calculateTheVolumeWaterCanReside(case4, case4.length));
        System.out.println("water that's filled will be == " + calculateTheVolumeWaterCanReside(case5, case5.length));


    }

    public static int calculateTheVolumeWaterCanReside(int[] arr, int n) {
        System.out.println("got array " + Arrays.toString(arr));
        int size = n - 1;//size for the loop

        int water = 0;//water stored

        // Let the previous wall height will be
        int prev = arr[0];

        // To store previous wall's index
        int prev_index = 0;

        //* Have a temporary variable that stores the same value as water.
        int temp = 0;


        for (int i = 1; i <= size; i++) {
//            System.out.println("*****************************************************************************************");
//            System.out.println("at index " + i + " current element is " + arr[i]);
//            System.out.println("current previous wall index = " + prev_index + "  current previous wall height = " + prev);
            // If the current wall is taller than
            // the previous wall then make current
            // wall as the previous wall and its
            // index as previous wall's index
            // for the subsequent loops
            if (arr[i] >= prev) {
//                System.out.println("current index greater than equal to previous wall index ");
                prev = arr[i];
                prev_index = i;

//                System.out.println("now changing the previous wall index =" + prev_index + "and previous wall height = " + prev);

                // Because larger or same height wall is found
                temp = 0;
            } else {

                //* Keep adding the previous wall’s height minus the current (ith) wall to the variable water.
                water += prev - arr[i];

//                System.out.println("adding the value for wall height - current wall to water = " + water);

                // Store the same value in temp as well
                // If we don't find any larger wall then
                // we will subtract temp from water
                temp += prev - arr[i];
            }
        }


        if (prev_index < size) {

            water -= temp;
//            System.out.println("No larger wall found so water have excess water so we remove it  " + water);


            prev = arr[size];
//            System.out.println("previous wall should be assigned to the last element" + prev);
//            System.out.println("looping through the end of array till the previous wall index which could have largest wall from left ");

            // Loop from the end of array up to the 'previous index' which was our previous wall height
            for (int i = size; i >= prev_index; i--) {


                // Right end wall should be smaller than prev index wall
                if (arr[i] >= prev) {
                    prev = arr[i];
//                    System.out.println("current array index is greater than equal to previous wall height so assigning it to be the new previous wall height \n new previous wall height is =  " + prev);
                } else {
                    water += prev - arr[i];
//                    System.out.println("keep adding the previous wall height - current index to the water " + water);

                }
            }
        }

        return water;
    }