# Largest Derangement of a Sequence

Given any sequence , find the largest derangement of .

A derangement is any permutation of , such that no two elements at the same position in and are equal.

The Largest Derangement is such that .

Examples:

```Input : seq[] = {5, 4, 3, 2, 1}
Output : 4 5 2 1 3

Input : seq[] = {56, 21, 42, 67, 23, 74}
Output : 74, 67, 56, 42, 23, 21
```

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

Since we are interested in generating largest derangement, we start putting larger elements in more significant positions.

Start from left, at any position place the next largest element among the values of the sequence which have not yet been placed in positions before .

To scan all positions takes N iteration. In each iteration we are required to find a maximum numbers, so a trivial implementation would be complexity,

However if we use a data structure like max-heap to find the maximum element, then the complexity reduces to

Below is C++ implementation.

 `// CPP program to find the largest derangement ` `#include ` `using` `namespace` `std; ` ` `  `void` `printLargest(``int` `seq[], ``int` `N) ` `{ ` `    ``int` `res[N]; ``// Stores result ` ` `  `    ``// Insert all elements into a priority queue ` `    ``std::priority_queue<``int``> pq;  ` `    ``for` `(``int` `i = 0; i < N; i++)  ` `        ``pq.push(seq[i]);     ` ` `  `    ``// Fill Up res[] from left to right ` `    ``for` `(``int` `i = 0; i < N; i++) { ` `        ``int` `d = pq.top(); ` `        ``pq.pop(); ` `        ``if` `(d != seq[i] || i == N - 1) { ` `            ``res[i] = d; ` `        ``} ``else` `{ ` ` `  `            ``// New Element poped equals the element  ` `            ``// in original sequence. Get the next ` `            ``// largest element ` `            ``res[i] = pq.top(); ` `            ``pq.pop(); ` `            ``pq.push(d); ` `        ``} ` `    ``} ` ` `  `    ``// If given sequence is in descending order then  ` `    ``// we need to swap last two elements again ` `    ``if` `(res[N - 1] == seq[N - 1]) { ` `        ``res[N - 1] = res[N - 2]; ` `        ``res[N - 2] = seq[N - 1]; ` `    ``} ` ` `  `    ``printf``(````" Largest Derangement "````); ` `    ``for` `(``int` `i = 0; i < N; i++)  ` `        ``printf``(``"%d "``, res[i]); ` `} ` ` `  `// Driver code ` `int` `main() ` `{ ` `    ``int` `seq[] = { 92, 3, 52, 13, 2, 31, 1 };  ` `    ``int` `n = ``sizeof``(seq)/``sizeof``(seq[0]); ` `    ``printLargest(seq, n); ` `    ``return` `0; ` `} `

Output:

```Sequence:
92 3 52 13 2 31 1
Largest Derangement
52 92 31 3 13 1 2
```

Note:
The method can be easily modified to obtain the smallest derangement as well.
Instead of a Max Heap, we should use a Min Heap to consecutively get minimum elements

Please write comments if you find anything incorrect, or you want to share more information about the topic discussed above.

This article is attributed to GeeksforGeeks.org

## tags:

Arrays Heap Arrays Heap

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