# Partition the array into three equal sum segments

Given an array of n integers, we have to partition the array into three segments such that all the segments have an equal sum. Segment sum is the sum of all the elements in the segment.

Examples:

```Input :  1, 3, 6, 2, 7, 1, 2, 8
Output : [1, 3, 6], [2, 7, 1], [2, 8]

Input :  7, 6, 1, 7
Output :  [7], [6, 1], [7]

Input :  7, 6, 2, 7
Output : Cannot divide the array into segments ```

A simple solution is to consider all pairs of indexes and, for every pair, check if it divides the array into three equal parts. If yes, then return true. The time complexity of this solution is O(n2)

An efficient approach is to use two auxiliary arrays and store the prefix and suffix array sum in these arrays respectively. We then use the two-pointer approach, with variable ‘i’ pointing to the start of the prefix array and variable ‘j’ pointing to the end of the suffix array. If pre[i] > suf[j], then decrement ‘j’, otherwise increment ‘i’.
We maintain a variable whose value is the total sum of the array and whenever we encounter pre[i] = total_sum / 3 or suf[j] = total_sum / 3, we store the value of i or j respectively as segment boundaries.

## C++

 `// C++ implementation of the approach``#include``using` `namespace` `std;` `// First segment's end index``static` `int` `pos1 = -1; ` `// Third segment's start index``static` `int` `pos2 = -1; ` `// This function returns true if the array ``// can be divided into three equal sum segments``bool` `equiSumUtil(``int` `arr[],``int` `n)``{` `    ``// Prefix Sum Array``    ``int` `pre[n];``    ``int` `sum = 0;``    ``for` `(``int` `i = 0; i < n; i++)``    ``{``        ``sum += arr[i];``        ``pre[i] = sum;``    ``}` `    ``// Suffix Sum Array``    ``int` `suf[n];``    ``sum = 0;``    ``for` `(``int` `i = n - 1; i >= 0; i--) ``    ``{``        ``sum += arr[i];``        ``suf[i] = sum;``    ``}` `    ``// Stores the total sum of the array``    ``int` `total_sum = sum; ` `    ``int` `i = 0, j = n - 1;``    ``while` `(i < j - 1) ``    ``{` `        ``if` `(pre[i] == total_sum / 3)``        ``{``                ``pos1 = i;``        ``}` `        ``if` `(suf[j] == total_sum / 3)``        ``{``            ``pos2 = j;``        ``}` `        ``if` `(pos1 != -1 && pos2 != -1) ``        ``{` `            ``// We can also take pre[pos2 - 1] - pre[pos1] ==``            ``// total_sum / 3 here.``            ``if` `(suf[pos1 + 1] - suf[pos2] == total_sum / 3) ``            ``{ ``                ``return` `true``;``            ``}``            ``else``            ``{``                ``return` `false``;``            ``}``        ``}` `        ``if` `(pre[i] < suf[j]) ``        ``{``            ``i++;``        ``}``        ``else``        ``{``            ``j--;``        ``}``    ``}` `    ``return` `false``;``}` `void` `equiSum(``int` `arr[],``int` `n)``{``    ``bool` `ans = equiSumUtil(arr,n);``    ``if` `(ans) ``    ``{` `        ``cout << ``"First Segment : "``;``        ``for` `(``int` `i = 0; i <= pos1; i++) ``        ``{``            ``cout << arr[i] << ``" "``;``        ``}` `        ``cout << endl;` `        ``cout << ``"Second Segment : "``;``        ``for` `(``int` `i = pos1 + 1; i < pos2; i++) ``        ``{``            ``cout << arr[i] << ``" "``;``        ``}` `        ``cout << endl;` `        ``cout << ``"Third Segment : "``;``        ``for` `(``int` `i = pos2; i < n; i++)``        ``{``            ``cout << arr[i] << ``" "``;``        ``}` `        ``cout<

## Java

 `public` `class` `Main {` `    ``//  First segment's end index``    ``public` `static` `int` `pos1 = -``1``; ` `    ``//  Third segment's start index``    ``public` `static` `int` `pos2 = -``1``; ` `    ``// This function returns true if the array ``    ``// can be divided into three equal sum segments``    ``public` `static` `boolean` `equiSumUtil(``int``[] arr)``    ``{``        ``int` `n = arr.length;` `        ``// Prefix Sum Array``        ``int``[] pre = ``new` `int``[n];``        ``int` `sum = ``0``;``        ``for` `(``int` `i = ``0``; i < n; i++) {``            ``sum += arr[i];``            ``pre[i] = sum;``        ``}` `        ``// Suffix Sum Array``        ``int``[] suf = ``new` `int``[n];``        ``sum = ``0``;``        ``for` `(``int` `i = n - ``1``; i >= ``0``; i--) {``            ``sum += arr[i];``            ``suf[i] = sum;``        ``}` `        ``// Stores the total sum of the array``        ``int` `total_sum = sum; ` `        ``int` `i = ``0``, j = n - ``1``;``        ``while` `(i < j - ``1``) {` `            ``if` `(pre[i] == total_sum / ``3``) {``                ``pos1 = i;``            ``}` `            ``if` `(suf[j] == total_sum / ``3``) {``                ``pos2 = j;``            ``}` `            ``if` `(pos1 != -``1` `&& pos2 != -``1``) {` `                ``// We can also take pre[pos2 - 1] - pre[pos1] ==``                ``// total_sum / 3 here.``                ``if` `(suf[pos1 + ``1``] - suf[pos2] == total_sum / ``3``) { ``                    ``return` `true``;``                ``}``                ``else` `{``                    ``return` `false``;``                ``}``            ``}` `            ``if` `(pre[i] < suf[j]) {``                ``i++;``            ``}``            ``else` `{``                ``j--;``            ``}``        ``}` `        ``return` `false``;``    ``}` `    ``public` `static` `void` `equiSum(``int``[] arr)``    ``{``        ``boolean` `ans = equiSumUtil(arr);``        ``if` `(ans) {` `            ``System.out.print(``"First Segment : "``);``            ``for` `(``int` `i = ``0``; i <= pos1; i++) {``                ``System.out.print(arr[i] + ``" "``);``            ``}` `            ``System.out.println();` `            ``System.out.print(``"Second Segment : "``);``            ``for` `(``int` `i = pos1 + ``1``; i < pos2; i++) {``                ``System.out.print(arr[i] + ``" "``);``            ``}` `            ``System.out.println();` `            ``System.out.print(``"Third Segment : "``);``            ``for` `(``int` `i = pos2; i < arr.length; i++) {``                ``System.out.print(arr[i] + ``" "``);``            ``}` `            ``System.out.println();``        ``}``        ``else` `{``            ``System.out.println(``"Array cannot be "` `+ ``            ``"divided into three equal sum segments"``);``        ``}``    ``}``    ``public` `static` `void` `main(String[] args)``    ``{``        ``int``[] arr = { ``1``, ``3``, ``6``, ``2``, ``7``, ``1``, ``2``, ``8` `};``        ``equiSum(arr);``    ``}``}`

## Python3

 `# Python3 implementation of the given approach` `# This function returns true if the array ``# can be divided into three equal sum segments``def` `equiSumUtil(arr, pos1, pos2):` `    ``n ``=` `len``(arr);` `    ``# Prefix Sum Array``    ``pre ``=` `[``0``] ``*` `n;``    ``sum` `=` `0``;``    ``for` `i ``in` `range``(n): ``        ``sum` `+``=` `arr[i];``        ``pre[i] ``=` `sum``;` `    ``# Suffix Sum Array``    ``suf ``=` `[``0``] ``*` `n;``    ``sum` `=` `0``;``    ``for` `i ``in` `range``(n ``-` `1``, ``-``1``, ``-``1``): ``        ``sum` `+``=` `arr[i];``        ``suf[i] ``=` `sum``;` `    ``# Stores the total sum of the array``    ``total_sum ``=` `sum``; ` `    ``i ``=` `0``;``    ``j ``=` `n ``-` `1``;``    ``while` `(i < j ``-` `1``): ` `        ``if` `(pre[i] ``=``=` `total_sum ``/``/` `3``): ``            ``pos1 ``=` `i;` `        ``if` `(suf[j] ``=``=` `total_sum ``/``/` `3``): ``            ``pos2 ``=` `j;` `        ``if` `(pos1 !``=` `-``1` `and` `pos2 !``=` `-``1``): ` `            ``# We can also take pre[pos2 - 1] - pre[pos1] ==``            ``# total_sum / 3 here.``            ``if` `(suf[pos1 ``+` `1``] ``-``                ``suf[pos2] ``=``=` `total_sum ``/``/` `3``):``                ``return` `[``True``, pos1, pos2];``            ``else``:``                ``return` `[``False``, pos1, pos2];` `        ``if` `(pre[i] < suf[j]): ``            ``i ``+``=` `1``;``        ``else``:``            ``j ``-``=` `1``;` `    ``return` `[``False``, pos1, pos2];` `def` `equiSum(arr):` `    ``pos1 ``=` `-``1``;``    ``pos2 ``=` `-``1``;``    ``ans ``=` `equiSumUtil(arr, pos1, pos2);``    ``pos1 ``=` `ans[``1``];``    ``pos2 ``=` `ans[``2``];``    ``if` `(ans[``0``]): ``        ``print``(``"First Segment : "``, end ``=` `"");``        ``for` `i ``in` `range``(pos1 ``+` `1``): ``            ``print``(arr[i], end ``=` `" "``);` `        ``print``("");` `        ``print``(``"Second Segment : "``, end ``=` `"");``        ``for` `i ``in` `range``(pos1 ``+` `1``, pos2): ``            ``print``(arr[i], end ``=` `" "``);` `        ``print``("");` `        ``print``(``"Third Segment : "``, end ``=` `"");``        ``for` `i ``in` `range``(pos2, ``len``(arr)): ``            ``print``(arr[i], end ``=` `" "``);` `        ``print``("");``    ``else``:``        ``println(``"Array cannot be divided into"``,``                ``"three equal sum segments"``);` `# Driver Code``arr ``=` `[``1``, ``3``, ``6``, ``2``, ``7``, ``1``, ``2``, ``8` `];``equiSum(arr);` `# This code is contributed by mits`

## C#

 `// C# implementation of the approach``using` `System;``    ` `class` `GFG ``{` `    ``// First segment's end index``    ``public` `static` `int` `pos1 = -1; ` `    ``// Third segment's start index``    ``public` `static` `int` `pos2 = -1; ` `    ``// This function returns true if the array ``    ``// can be divided into three equal sum segments``    ``public` `static` `bool` `equiSumUtil(``int``[] arr)``    ``{``        ``int` `n = arr.Length;` `        ``// Prefix Sum Array``        ``int``[] pre = ``new` `int``[n];``        ``int` `sum = 0,i;``        ``for` `(i = 0; i < n; i++) ``        ``{``            ``sum += arr[i];``            ``pre[i] = sum;``        ``}` `        ``// Suffix Sum Array``        ``int``[] suf = ``new` `int``[n];``        ``sum = 0;``        ``for` `(i = n - 1; i >= 0; i--)``        ``{``            ``sum += arr[i];``            ``suf[i] = sum;``        ``}` `        ``// Stores the total sum of the array``        ``int` `total_sum = sum; ` `        ``int` `j = n - 1;``        ``i = 0;``        ``while` `(i < j - 1) ``        ``{` `            ``if` `(pre[i] == total_sum / 3) ``            ``{``                ``pos1 = i;``            ``}` `            ``if` `(suf[j] == total_sum / 3) ``            ``{``                ``pos2 = j;``            ``}` `            ``if` `(pos1 != -1 && pos2 != -1) ``            ``{` `                ``// We can also take pre[pos2 - 1] - pre[pos1] ==``                ``// total_sum / 3 here.``                ``if` `(suf[pos1 + 1] - suf[pos2] == total_sum / 3) ``                ``{ ``                    ``return` `true``;``                ``}``                ``else``                ``{``                    ``return` `false``;``                ``}``            ``}` `            ``if` `(pre[i] < suf[j]) ``            ``{``                ``i++;``            ``}``            ``else``            ``{``                ``j--;``            ``}``        ``}` `        ``return` `false``;``    ``}` `    ``public` `static` `void` `equiSum(``int``[] arr)``    ``{``        ``bool` `ans = equiSumUtil(arr);``        ``if` `(ans)``        ``{` `            ``Console.Write(``"First Segment : "``);``            ``for` `(``int` `i = 0; i <= pos1; i++)``            ``{``                ``Console.Write(arr[i] + ``" "``);``            ``}` `            ``Console.WriteLine();` `            ``Console.Write(``"Second Segment : "``);``            ``for` `(``int` `i = pos1 + 1; i < pos2; i++) ``            ``{``                ``Console.Write(arr[i] + ``" "``);``            ``}` `            ``Console.WriteLine();` `            ``Console.Write(``"Third Segment : "``);``            ``for` `(``int` `i = pos2; i < arr.Length; i++) ``            ``{``                ``Console.Write(arr[i] + ``" "``);``            ``}` `            ``Console.WriteLine();``        ``}``        ``else``        ``{``            ``Console.WriteLine(``"Array cannot be "` `+ ``            ``"divided into three equal sum segments"``);``        ``}``    ``}``    ``public` `static` `void` `Main(String[] args)``    ``{``        ``int``[] arr = { 1, 3, 6, 2, 7, 1, 2, 8 };``        ``equiSum(arr);``    ``}``}` `// This code contributed by Rajput-Ji`

## PHP

 `= 0; ``\$i``--) ``    ``{``        ``\$sum` `+= ``\$arr``[``\$i``];``        ``\$suf``[``\$i``] = ``\$sum``;``    ``}` `    ``// Stores the total sum of the array``    ``\$total_sum` `= ``\$sum``; ` `    ``\$i` `= 0;``    ``\$j` `= ``\$n` `- 1;``    ``while` `(``\$i` `< ``\$j` `- 1) ``    ``{` `        ``if` `(``\$pre``[``\$i``] == ``\$total_sum` `/ 3) ``        ``{``            ``\$pos1` `= ``\$i``;``        ``}` `        ``if` `(``\$suf``[``\$j``] == ``\$total_sum` `/ 3) ``        ``{``            ``\$pos2` `= ``\$j``;``        ``}` `        ``if` `(``\$pos1` `!= -1 && ``\$pos2` `!= -1) ``        ``{` `            ``// We can also take pre[pos2 - 1] - pre[pos1] ==``            ``// total_sum / 3 here.``            ``if` `(``\$suf``[``\$pos1` `+ 1] - ``                     ``\$suf``[``\$pos2``] == ``\$total_sum` `/ 3)``            ``{ ``                ``return` `true;``            ``}``            ``else``            ``{``                ``return` `false;``            ``}``        ``}` `        ``if` `(``\$pre``[``\$i``] < ``\$suf``[``\$j``]) ``        ``{``            ``\$i``++;``        ``}``        ``else``        ``{``            ``\$j``--;``        ``}``    ``}` `    ``return` `false;``}` `function` `equiSum(``\$arr``)``{``    ``global` `\$pos2``,``\$pos1``;``    ``\$ans` `= equiSumUtil(``\$arr``);``    ``if` `(``\$ans``) ``    ``{` `        ``print``(``"First Segment : "``);``        ``for` `(``\$i` `= 0; ``\$i` `<= ``\$pos1``; ``\$i``++) ``        ``{``            ``print``(``\$arr``[``\$i``] . ``" "``);``        ``}` `        ``print``(``"\n"``);` `        ``print``(``"Second Segment : "``);``        ``for` `(``\$i` `= ``\$pos1` `+ 1; ``\$i` `< ``\$pos2``; ``\$i``++) ``        ``{``            ``print``(``\$arr``[``\$i``] . ``" "``);``        ``}` `        ``print``(``"\n"``);` `        ``print``(``"Third Segment : "``);``        ``for` `(``\$i` `= ``\$pos2``; ``\$i` `< ``count``(``\$arr``); ``\$i``++) ``        ``{``            ``print``(``\$arr``[``\$i``] . ``" "``);``        ``}` `        ``print``(``"\n"``);``    ``}``    ``else``    ``{``        ``println(``"Array cannot be divided into "``, ``                ``"three equal sum segments"``);``    ``}``}` `// Driver Code``\$arr` `= ``array``(1, 3, 6, 2, 7, 1, 2, 8 );``equiSum(``\$arr``);` `// This code is contributed by mits``?>`

## Javascript

 ``

Output:
```First Segment : 1 3 6
Second Segment : 2 7 1
Third Segment : 2 8```

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

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