# Minimum time to finish tasks without skipping two consecutive

Given time taken by n tasks. Find the minimum time needed to finish the tasks such that skipping of tasks is allowed, but can not skip two consecutive tasks.

Examples :

Input : arr[] = {10, 5, 7, 10} Output : 12 We can skip first and last task and finish these task in 12 min. Input : arr[] = {10} Output : 0 There is only one task and we can skip it. Input : arr[] = {10, 30} Output : 10 Input : arr[] = {10, 5, 2, 4, 8, 6, 7, 10} Output : 22

Expected Time Complexity is O(n) and extra space is O(1).

The given problem has the following recursive property.

Let **minTime(i)** be minimum time to finish till i’th task. It can be written as minimum of two values.

- Minimum time if i’th task is included in list, let this time be incl(i)
- Minimum time if i’th task is excluded from result, let this time be excl(i)

minTime(i) = min(excl(i), incl(i))

Result is **minTime(n-1)** if there are n tasks and indexes start from 0.

**incl(i)** can be written as below.

// There are two possibilities // (a) Previous task is also included // (b) Previous task is not included incl(i) = min(incl(i-1), excl(i-1)) + arr[i] // Since this is inclusive // arr[i] must be included

**excl(i)** can be written as below.

// There is only one possibility (Previous task must be // included as we can't skip consecutive tasks. excl(i) = incl(i-1)

A simple solution is to make two tables incl[] and excl[] to store times for tasks. Finally return minimum of incl[n-1] and excl[n-1]. This solution requires O(n) time and O(n) space.

If we take a closer look, we can notice that we only need incl and excl of previous job. So we can save space and solve the problem in O(n) time and O(1) space. Below is C++ implementation of the idea.

## C++

`// C++ program to find minimum time to finish tasks ` `// such that no two consecutive tasks are skipped. ` `#include <bits/stdc++.h> ` `using` `namespace` `std; ` ` ` `// arr[] represents time taken by n given tasks ` `int` `minTime(` `int` `arr[], ` `int` `n) ` `{ ` ` ` `// Corner Cases ` ` ` `if` `(n <= 0) ` ` ` `return` `0; ` ` ` ` ` `// Initialize value for the case when there ` ` ` `// is only one task in task list. ` ` ` `int` `incl = arr[0]; ` `// First task is included ` ` ` `int` `excl = 0; ` `// First task is exluded ` ` ` ` ` `// Process remaining n-1 tasks ` ` ` `for` `(` `int` `i=1; i<n; i++) ` ` ` `{ ` ` ` `// Time taken if current task is included ` ` ` `// There are two possibilities ` ` ` `// (a) Previous task is also included ` ` ` `// (b) Previous task is not included ` ` ` `int` `incl_new = arr[i] + min(excl, incl); ` ` ` ` ` `// Time taken when current task is not ` ` ` `// included. There is only one possibility ` ` ` `// that previous task is also included. ` ` ` `int` `excl_new = incl; ` ` ` ` ` `// Update incl and excl for next iteration ` ` ` `incl = incl_new; ` ` ` `excl = excl_new; ` ` ` `} ` ` ` ` ` `// Return maximum of two values for last task ` ` ` `return` `min(incl, excl); ` `} ` ` ` `// Driver code ` `int` `main() ` `{ ` ` ` `int` `arr1[] = {10, 5, 2, 7, 10}; ` ` ` `int` `n1 = ` `sizeof` `(arr1)/` `sizeof` `(arr1[0]); ` ` ` `cout << minTime(arr1, n1) << endl; ` ` ` ` ` `int` `arr2[] = {10, 5, 7, 10}; ` ` ` `int` `n2 = ` `sizeof` `(arr2)/` `sizeof` `(arr2[0]); ` ` ` `cout << minTime(arr2, n2) << endl; ` ` ` ` ` `int` `arr3[] = {10, 5, 2, 4, 8, 6, 7, 10}; ` ` ` `int` `n3 = ` `sizeof` `(arr3)/` `sizeof` `(arr3[0]); ` ` ` `cout << minTime(arr3, n3) << endl; ` ` ` ` ` `return` `0; ` `} ` |

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## Java

`// Java program to find minimum time to ` `// finish tasks such that no two ` `// consecutive tasks are skipped. ` `import` `java.io.*; ` ` ` `class` `GFG { ` ` ` ` ` `// arr[] represents time taken by n ` ` ` `// given tasks ` ` ` `static` `int` `minTime(` `int` `arr[], ` `int` `n) ` ` ` `{ ` ` ` `// Corner Cases ` ` ` `if` `(n <= ` `0` `) ` ` ` `return` `0` `; ` ` ` ` ` `// Initialize value for the case ` ` ` `// when there is only one task in ` ` ` `// task list. ` ` ` `// First task is included ` ` ` `int` `incl = arr[` `0` `]; ` ` ` ` ` `// First task is exluded ` ` ` `int` `excl = ` `0` `; ` ` ` ` ` `// Process remaining n-1 tasks ` ` ` `for` `(` `int` `i = ` `1` `; i < n; i++) ` ` ` `{ ` ` ` `// Time taken if current task is ` ` ` `// included. There are two ` ` ` `// possibilities ` ` ` `// (a) Previous task is also included ` ` ` `// (b) Previous task is not included ` ` ` `int` `incl_new = arr[i] + Math.min(excl, ` ` ` `incl); ` ` ` ` ` `// Time taken when current task is not ` ` ` `// included. There is only one ` ` ` `// possibility that previous task is ` ` ` `// also included. ` ` ` `int` `excl_new = incl; ` ` ` ` ` `// Update incl and excl for next ` ` ` `// iteration ` ` ` `incl = incl_new; ` ` ` `excl = excl_new; ` ` ` `} ` ` ` ` ` `// Return maximum of two values for ` ` ` `// last task ` ` ` `return` `Math.min(incl, excl); ` ` ` `} ` ` ` ` ` `// Driver code ` ` ` `public` `static` `void` `main(String[] args) ` ` ` `{ ` ` ` `int` `arr1[] = {` `10` `, ` `5` `, ` `2` `, ` `7` `, ` `10` `}; ` ` ` `int` `n1 = arr1.length; ` ` ` `System.out.println(minTime(arr1, n1)); ` ` ` ` ` `int` `arr2[] = {` `10` `, ` `5` `, ` `7` `, ` `10` `}; ` ` ` `int` `n2 = arr2.length; ` ` ` `System.out.println(minTime(arr2, n2)); ` ` ` ` ` `int` `arr3[] = {` `10` `, ` `5` `, ` `2` `, ` `4` `, ` `8` `, ` `6` `, ` `7` `, ` `10` `}; ` ` ` `int` `n3 = arr3.length; ` ` ` `System.out.println(minTime(arr3, n3)); ` ` ` ` ` `} ` `} ` `// This code is contributed by Prerna Saini ` |

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## Python3

` ` `# Python3 program to find minimum ` `# time to finish tasks such that no ` `# two consecutive tasks are skipped. ` ` ` `# arr[] represents time ` `# taken by n given tasks ` `def` `minTime(arr, n): ` ` ` ` ` `# Corner Cases ` ` ` `if` `(n <` `=` `0` `): ` `return` `0` ` ` ` ` `# Initialize value for the ` ` ` `# case when there is only ` ` ` `# one task in task list. ` ` ` `incl ` `=` `arr[` `0` `] ` `# First task is included ` ` ` `excl ` `=` `0` `# First task is exluded ` ` ` ` ` `# Process remaining n-1 tasks ` ` ` `for` `i ` `in` `range` `(` `1` `, n): ` ` ` ` ` `# Time taken if current task is included ` ` ` `# There are two possibilities ` ` ` `# (a) Previous task is also included ` ` ` `# (b) Previous task is not included ` ` ` `incl_new ` `=` `arr[i] ` `+` `min` `(excl, incl) ` ` ` ` ` `# Time taken when current task is not ` ` ` `# included. There is only one possibility ` ` ` `# that previous task is also included. ` ` ` `excl_new ` `=` `incl ` ` ` ` ` `# Update incl and excl for next iteration ` ` ` `incl ` `=` `incl_new ` ` ` `excl ` `=` `excl_new ` ` ` ` ` ` ` `# Return maximum of two values for last task ` ` ` `return` `min` `(incl, excl) ` ` ` `# Driver code ` `arr1 ` `=` `[` `10` `, ` `5` `, ` `2` `, ` `7` `, ` `10` `] ` `n1 ` `=` `len` `(arr1) ` `print` `(minTime(arr1, n1)) ` ` ` `arr2 ` `=` `[` `10` `, ` `5` `, ` `7` `, ` `10` `] ` `n2 ` `=` `len` `(arr2) ` `print` `(minTime(arr2, n2)) ` ` ` `arr3 ` `=` `[` `10` `, ` `5` `, ` `2` `, ` `4` `, ` `8` `, ` `6` `, ` `7` `, ` `10` `] ` `n3 ` `=` `len` `(arr3) ` `print` `(minTime(arr3, n3)) ` ` ` `# This code is contributed by Anant Agarwal. ` |

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## C#

`// C# program to find minimum time to ` `// finish tasks such that no two ` `// consecutive tasks are skipped. ` `using` `System; ` ` ` `class` `GFG { ` ` ` ` ` `// arr[] represents time taken by n ` ` ` `// given tasks ` ` ` `static` `int` `minTime(` `int` `[]arr, ` `int` `n) ` ` ` `{ ` ` ` `// Corner Cases ` ` ` `if` `(n <= 0) ` ` ` `return` `0; ` ` ` ` ` `// Initialize value for the case ` ` ` `// when there is only one task in ` ` ` `// task list. ` ` ` `// First task is included ` ` ` `int` `incl = arr[0]; ` ` ` ` ` `// First task is exluded ` ` ` `int` `excl = 0; ` ` ` ` ` `// Process remaining n-1 tasks ` ` ` `for` `(` `int` `i = 1; i < n; i++) ` ` ` `{ ` ` ` `// Time taken if current task is ` ` ` `// included. There are two ` ` ` `// possibilities ` ` ` `// (a) Previous task is also included ` ` ` `// (b) Previous task is not included ` ` ` `int` `incl_new = arr[i] + Math.Min(excl, ` ` ` `incl); ` ` ` ` ` `// Time taken when current task is not ` ` ` `// included. There is only one ` ` ` `// possibility that previous task is ` ` ` `// also included. ` ` ` `int` `excl_new = incl; ` ` ` ` ` `// Update incl and excl for next ` ` ` `// iteration ` ` ` `incl = incl_new; ` ` ` `excl = excl_new; ` ` ` `} ` ` ` ` ` `// Return maximum of two values for ` ` ` `// last task ` ` ` `return` `Math.Min(incl, excl); ` ` ` `} ` ` ` ` ` `// Driver code ` ` ` `public` `static` `void` `Main() ` ` ` `{ ` ` ` `int` `[]arr1 = {10, 5, 2, 7, 10}; ` ` ` `int` `n1 = arr1.Length; ` ` ` `Console.WriteLine(minTime(arr1, n1)); ` ` ` ` ` `int` `[]arr2 = {10, 5, 7, 10}; ` ` ` `int` `n2 = arr2.Length; ` ` ` `Console.WriteLine(minTime(arr2, n2)); ` ` ` ` ` `int` `[]arr3 = {10, 5, 2, 4, 8, 6, 7, 10}; ` ` ` `int` `n3 = arr3.Length; ` ` ` `Console.WriteLine(minTime(arr3, n3)); ` ` ` ` ` `} ` `} ` `// This code is contributed by Anant Agarwal. ` |

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## PHP

`<?php ` `// PHP program to find minimum time ` `// to finish tasks such that no two ` `// consecutive tasks are skipped. ` ` ` `// arr[] represents time ` `// taken by n given tasks ` `function` `minTime(` `$arr` `, ` `$n` `) ` `{ ` ` ` `// Corner Cases ` ` ` `if` `(` `$n` `<= 0) ` ` ` `return` `0; ` ` ` ` ` `// Initialize value for the ` ` ` `// case when there is only ` ` ` `// one task in task list. ` ` ` ` ` `// First task is included ` ` ` `$incl` `= ` `$arr` `[0]; ` ` ` ` ` `// First task is exluded ` ` ` `$excl` `= 0; ` ` ` ` ` `// Process remaining n-1 tasks ` ` ` `for` `(` `$i` `= 1; ` `$i` `< ` `$n` `; ` `$i` `++) ` ` ` `{ ` ` ` `// Time taken if current task is ` ` ` `// included There are two possibilities ` ` ` `// (a) Previous task is also included ` ` ` `// (b) Previous task is not included ` ` ` `$incl_new` `= ` `$arr` `[` `$i` `] + min(` `$excl` `, ` `$incl` `); ` ` ` ` ` `// Time taken when current task ` ` ` `// is not included. There is only ` ` ` `// one possibility that previous ` ` ` `// task is also included. ` ` ` `$excl_new` `= ` `$incl` `; ` ` ` ` ` `// Update incl and excl ` ` ` `// for next iteration ` ` ` `$incl` `= ` `$incl_new` `; ` ` ` `$excl` `= ` `$excl_new` `; ` ` ` `} ` ` ` ` ` `// Return maximum of two ` ` ` `// values for last task ` ` ` `return` `min(` `$incl` `, ` `$excl` `); ` `} ` ` ` `// Driver code ` ` ` `$arr1` `= ` `array` `(10, 5, 2, 7, 10); ` `$n1` `= sizeof(` `$arr1` `); ` `echo` `minTime(` `$arr1` `, ` `$n1` `),` `"\n"` `; ` ` ` `$arr2` `= ` `array` `(10, 5, 7, 10); ` `$n2` `= sizeof(` `$arr2` `); ` `echo` `minTime(` `$arr2` `, ` `$n2` `),` `"\n"` `; ` ` ` `$arr3` `= ` `array` `(10, 5, 2, 4, ` ` ` `8, 6, 7, 10); ` `$n3` `= sizeof(` `$arr3` `); ` `echo` `minTime(` `$arr3` `, ` `$n3` `); ` ` ` `// This code is contributed ` `// by nitin mittal. ` `?> ` |

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**Output :**

12 12 22

**Related Problems:**

Find minimum time to finish all jobs with given constraints

Maximum sum such that no two elements are adjacent.

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

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