Minimum increment or decrement required to sort the array | Top-down Approach

Given an array arr[] of N integers, the task is to sort the array in increasing order by performing minimum number of operations. In a single operation, an element of the array can either be incremented or decremented by 1. Print the minimum number of operations required.


Input: arr[] = {5, 4, 3, 2, 1}
Output: 6
The sorted array of arr[] is {3, 3, 3, 3, 3}
Therefore the minimum increments/decrement are:
At index 0, 5 – 3 = 2 (decrement 2)
At index 1, 4 – 3 = 1 (decrement 1)
At index 3, 2 + 1 = 3 (increment 1)
At index 4, 1 + 2 = 3 (increment 2)
The total increment/decrement is 2 + 1 + 1 + 2 = 6.

Input: arr[] = {1, 2, 3, 4}
Output: 0
The array is already sorted.

Bottom-up Approach: This problem can be solved using Dynamic Programming. A Bottom-up Approach to this problem statement is discussed in this article.

Bottom-up Approach: Here we will use Top-down Dynamic Programming to solve this problem.

Let 2D array (say dp[i][j]) used to store the upto index i where last element is at index j.

Below are the steps:

  1. To make the array element in sorted by using the given operations, we know that an element cannot become greater than the maximum value of the array and less than the minimum value of the array(say m) by increment or decrement.
  2. Therefore, Fix an element(say X) at ith position, then (i-1)th position value(say Y) can be in the range [m, X].
  3. Keep placing the smaller element less than or equals to arr[i] at (i-1)th position for every index i of arr[] and calculate the minimum increment or decrement by adding abs(arr[i] – Y).
  4. Therefore the recurrence relation for the above mentioned approach can be written as:

    dp[i][j] = min(dp[i][j], abs(arr[i] - Y) + recursive_function(i-1, Y))

    where m ≤ Y ≤ arr[j].

Below is the implementation of the above approach:





// C++ program of the above approach
#include <bits/stdc++.h>
using namespace std;
// Dp array to memoized
// the value recursive call
int dp[1000][1000];
// Function to find the minimum increment
// or decrement needed to make the array
// sorted
int minimumIncDec(int arr[], int N,
                  int maxE, int minE)
    // If only one element is present,
    // then arr[] is sorted
    if (N == 0) {
        return 0;
    // If dp[N][maxE] is precalculated,
    // then return the result
    if (dp[N][maxE])
        return dp[N][maxE];
    int ans = INT_MAX;
    // Iterate from minE to maxE which
    // placed at previous index
    for (int k = minE; k <= maxE; k++) {
        // Update the answer according to
        // recurrence relation
        int x = minimumIncDec(
            arr, N - 1, k, minE);
        ans = min(
            x + abs(arr[N - 1] - k));
    // Memoized the value
    // for dp[N][maxE]
    dp[N][maxE] = ans;
    // Return the final result
    return dp[N][maxE];
// Driver Code
int main()
    int arr[] = { 5, 4, 3, 2, 1 };
    int N = sizeof(arr) / sizeof(arr[0]);
    // Find the minimum and maximum
    // element from the arr[]
    int minE = *min_element(arr, arr + N);
    int maxE = *max_element(arr, arr + N);
    // Function Call
    cout << minimumIncDec(
        arr, N, maxE, minE);
    return 0;




Time Complexity: O(N2)
Auxiliary Space: O(N2)

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