Maximum subsequence sum such that no three are consecutive

Given a sequence of positive numbers, find the maximum sum that can be formed which has no three consecutive elements present.

Examples:

```Input: arr[] = {1, 2, 3}
Output: 5
We can't take three of them, so answer is
2 + 3 = 5

Input: arr[] = {3000, 2000, 1000, 3, 10}
Output: 5013
3000 + 2000 + 3 + 10 = 5013

Input: arr[] = {100, 1000, 100, 1000, 1}
Output: 2101
100 + 1000 + 1000 + 1 = 2101

Input: arr[] = {1, 1, 1, 1, 1}
Output: 4

Input: arr[] = {1, 2, 3, 4, 5, 6, 7, 8}
Output: 27
```

Recommended: Please try your approach on {IDE} first, before moving on to the solution.

This problem is mainly an extension of below problem.

Maximum sum such that no two elements are adjacent

We maintain an auxiliary array sum[] (of same size as input array) to find the result.

```sum[i] : Stores result for subarray arr[0..i], i.e.,
maximum possible sum in subarray arr[0..i]
such that no three elements are consecutive.

sum[0] = arr[0]

// Note : All elements are positive
sum[1] = arr[0] + arr[1]

// We have three cases
// 1) Exclude arr[2], i.e., sum[2] = sum[1]
// 2) Exclude arr[1], i.e., sum[2] = sum[0] + arr[2]
// 3) Exclude arr[0], i.e., sum[2] = arr[1] + arr[2]
sum[2] = max(sum[1], arr[0] + arr[2], arr[1] + arr[2])

In general,
// We have three cases
// 1) Exclude arr[i],  i.e.,  sum[i] = sum[i-1]
// 2) Exclude arr[i-1], i.e., sum[i] = sum[i-2] + arr[i]
// 3) Exclude arr[i-2], i.e., sum[i-3] + arr[i] + arr[i-1]
sum[i] = max(sum[i-1], sum[i-2] + arr[i],
sum[i-3] + arr[i] + arr[i-1])
```

Below is implementation of above idea.

C/C++

```// C++ program to find the maximum sum such that
// no three are consecutive
#include<bits/stdc++.h>
using namespace std;

// Returns maximum subsequence sum such that no three
// elements are consecutive
int maxSumWO3Consec(int arr[], int n)
{
// Stores result for subarray arr[0..i], i.e.,
// maximum possible sum in subarray arr[0..i]
// such that no three elements are consecutive.
int sum[n];

// Base cases (process first three elements)
sum[0] = arr[0];
sum[1] = arr[0] + arr[1];
sum[2] = max(sum[1], arr[1] + arr[2]);

// Process rest of the elements
// We have three cases
// 1) Exclude arr[i],  i.e.,  sum[i] = sum[i-1]
// 2) Exclude arr[i-1], i.e., sum[i] = sum[i-2] + arr[i]
// 3) Exclude arr[i-2], i.e., sum[i-3] + arr[i] + arr[i-1]
for (int i=3; i<n; i++)
sum[i] = max(max(sum[i-1], sum[i-2] + arr[i]),
arr[i] + arr[i-1] + sum[i-3]);

return sum[n-1];
}

// Driver code
int main()
{
int arr[] = {100, 1000, 100, 1000, 1};
int n = sizeof(arr)/sizeof(arr[0]);
cout << maxSumWO3Consec(arr, n);
return 0;
}
```

Python

```# Python program to find the maximum sum such that
# no three are consecutive

# Returns maximum subsequence sum such that no three
# elements are consecutive
def maxSumWO3Consec(arr, n):
# Stores result for subarray arr[0..i], i.e.,
# maximum possible sum in subarray arr[0..i]
# such that no three elements are consecutive.
sum = [0 for k in range(n)]

# Base cases (process first three elements)
sum[0] = arr[0]
sum[1] = arr[0] + arr[1]
sum[2] = max(sum[1], arr[1] + arr[2])

# Process rest of the elements
# We have three cases
# 1) Exclude arr[i],  i.e.,  sum[i] = sum[i-1]
# 2) Exclude arr[i-1], i.e., sum[i] = sum[i-2] + arr[i]
# 3) Exclude arr[i-2], i.e., sum[i-3] + arr[i] + arr[i-1]
for i in range(3,n):
sum[i] = max(max(sum[i-1], sum[i-2] + arr[i]),arr[i] + arr[i-1] + sum[i-3])

return sum[n-1]

# Driver code
arr = [100, 1000, 100, 1000, 1]
n = len(arr)
print maxSumWO3Consec(arr, n)

#This code is contributed by Afzal Ansari
```

Output:

```2101
```

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

Exercise : Optimize above solution so that extra space required is O(1)

This article is contributed by Roshni Agarwal. If you like GeeksforGeeks and would like to contribute, you can also write an article using contribute.geeksforgeeks.org or mail your article to contribute@geeksforgeeks.org. See your article appearing on the GeeksforGeeks main page and help other Geeks.

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