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Minimize cost to reach end of an array by two forward jumps or one backward jump in each move
  • Difficulty Level : Expert
  • Last Updated : 09 Mar, 2021
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Given an array arr[] consisting of N positive integers, the task is to find the minimum cost required to either cross the array or reach the end of the array by only moving to indices (i + 2) and (i – 1) from the ith index.

Examples:

Input: arr[] = {5, 1, 2, 10, 100}
Output: 18
Explanation:
Optimal cost path (0 based indexing): 0 → 2 → 1 → 3 → 5
Therefore, the minimum cost = 5 + 2 + 1 + 10 = 18.

Input: arr[] = {9, 4, 6, 8, 5}
Output: 20
Explanation:
Optimal cost path (0 based indexing): 0 → 2 → 4
Therefore, the minimum cost = 9 + 6 + 5 = 20

Naive Approach: The given problem can be solved based on the following observations:



  • Since all costs are positive, it will never be an optimal option to move more than one step backward, hence to reach a particular index i of the array, either jump directly from the (i – 2)th index or jump from (i – 1)th to (i + 1)th index, i.e. (2 jumps forward), followed by 1 backward jump, i.e. from (i + 1)th index to ith index.
  • Now, traverse from the end of the array recursively and for the elements at indices (i – 2) and (i – 1), calculate the minimum cost of the two. Therefore, the minimum cost to cross the array can be calculated using the following recurrence relation:

minCost(index) = minimum(minCost(index – 2) + arr[i], minCost(index – 1) + arr[i] + arr[i + 1])

Time Complexity: O(2N)
Auxiliary Space: O(1)

Efficient Approach: The approach discussed above has both Optimal Substructure and Overlapping Subproblems. Therefore it can be optimized by either using Memoization or Tabulation. Follow the steps below to solve the problem:

  • Initialize an array dp[], where dp[i] stores the minimum cost to reach the ith index.
  • Initialize dp[0] = arr[0] as the cost to reach the 0th index, which is equal to the value at the 0th index itself. Update dp[1] = arr[0] + arr[1] + arr[2], as to reach the 1st index, jump from the 0th index to 2nd index indexn to the 1st index.
  • Iterate over the range [2, N – 2] using a variable i and update dp[i] as the minimum of (dp[i – 2] + arr[i]) and (dp[i – 1] + arr[i] + arr[i + 1]).
  • For the last index (N – 1), update dp[N – 1] as minimum of (dp[N – 3] + arr[N – 1]) and (dp[N – 2]).
  • After completing the above steps, print the value of dp[N – 1] as the result.

Below is the implementation of the above approach:

C++




// C++ program for the above approach
#include <bits/stdc++.h>
using namespace std;
 
// Function to find the minimum cost
// to reach the end of an array
void minCost(int arr[], int n)
{
    // Base Case: When N < 3
    if (n < 3) {
        cout << arr[0];
        return;
    }
 
    // Store the results in table
    int* dp = new int[n];
 
    // Initialize base cases
    dp[0] = arr[0];
    dp[1] = dp[0] + arr[1] + arr[2];
 
    // Iterate over the range[2, N - 2]
    // to construct the dp array
    for (int i = 2; i < n - 1; i++)
        dp[i] = min(dp[i - 2] + arr[i],
                    dp[i - 1] + arr[i]
                        + arr[i + 1]);
 
    // Handle case for the last index, i.e. N - 1
    dp[n - 1] = min(dp[n - 2],
                    dp[n - 3] + arr[n - 1]);
 
    // Print the answer
    cout << dp[n - 1];
}
 
// Driver Code
int main()
{
    int arr[] = { 9, 4, 6, 8, 5 };
    int N = sizeof(arr) / sizeof(arr[0]);
    minCost(arr, N);
 
    return 0;
}

Java




// Java Program to implement
// the above approach
import java.io.*;
import java.util.*;
 
class GFG
{
 
    // Function to find the minimum cost
    // to reach the end of an array
    static void minCost(int arr[], int n)
    {
       
        // Base Case: When N < 3
        if (n < 3) {
            System.out.println(arr[0]);
            return;
        }
 
        // Store the results in table
        int dp[] = new int[n];
 
        // Initialize base cases
        dp[0] = arr[0];
        dp[1] = dp[0] + arr[1] + arr[2];
 
        // Iterate over the range[2, N - 2]
        // to construct the dp array
        for (int i = 2; i < n - 1; i++)
            dp[i] = Math.min(dp[i - 2] + arr[i],
                           dp[i - 1] + arr[i] + arr[i + 1]);
 
        // Handle case for the last index, i.e. N - 1
        dp[n - 1] = Math.min(dp[n - 2], dp[n - 3] + arr[n - 1]);
 
        // Print the answer
        System.out.println(dp[n - 1]);
    }
 
    // Driver Code
    public static void main(String[] args)
    {
        int arr[] = { 9, 4, 6, 8, 5 };
        int N = arr.length;
        minCost(arr, N);
    }
}
 
// This code is contributed by Kingash.

Python3




# Python 3 program for the above approach
 
# Function to find the minimum cost
# to reach the end of an array
def minCost(arr, n):
 
    # Base Case: When N < 3
    if (n < 3):
        print(arr[0])
        return
 
    # Store the results in table
    dp = [0] * n
 
    # Initialize base cases
    dp[0] = arr[0]
    dp[1] = dp[0] + arr[1] + arr[2]
 
    # Iterate over the range[2, N - 2]
    # to construct the dp array
    for i in range(2, n - 1):
        dp[i] = min(dp[i - 2] + arr[i],
                    dp[i - 1] + arr[i]
                    + arr[i + 1])
 
    # Handle case for the last index, i.e. N - 1
    dp[n - 1] = min(dp[n - 2],
                    dp[n - 3] + arr[n - 1])
 
    # Print the answer
    print(dp[n - 1])
 
# Driver Code
if __name__ == "__main__":
 
    arr = [9, 4, 6, 8, 5]
    N = len(arr)
    minCost(arr, N)
 
    # This code is contributed by ukasp.

C#




// C# Program to implement
// the above approach
using System;
public class GFG
{
 
  // Function to find the minimum cost
  // to reach the end of an array
  static void minCost(int []arr, int n)
  {
 
    // Base Case: When N < 3
    if (n < 3) {
      Console.WriteLine(arr[0]);
      return;
    }
 
    // Store the results in table
    int []dp = new int[n];
 
    // Initialize base cases
    dp[0] = arr[0];
    dp[1] = dp[0] + arr[1] + arr[2];
 
    // Iterate over the range[2, N - 2]
    // to construct the dp array
    for (int i = 2; i < n - 1; i++)
      dp[i] = Math.Min(dp[i - 2] + arr[i],
                       dp[i - 1] + arr[i] + arr[i + 1]);
 
    // Handle case for the last index, i.e. N - 1
    dp[n - 1] = Math.Min(dp[n - 2], dp[n - 3] + arr[n - 1]);
 
    // Print the answer
    Console.WriteLine(dp[n - 1]);
  }
 
  // Driver Code
  public static void Main(string[] args)
  {
    int []arr = { 9, 4, 6, 8, 5 };
    int N = arr.Length;
    minCost(arr, N);
  }
}
 
// This code is contributed by AnkThon
Output: 
20

 

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

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