|
| 1 | +/** |
| 2 | + * @file |
| 3 | + * @brief Quick Sort without recursion. This method uses the stack instead. |
| 4 | + * Both recursive and iterative implementations have O(n log n) best case |
| 5 | + * and O(n^2) worst case. |
| 6 | + * @details |
| 7 | + * https://stackoverflow.com/questions/12553238/quicksort-iterative-or-recursive |
| 8 | + * https://en.wikipedia.org/wiki/Quicksort |
| 9 | + * https://www.geeksforgeeks.org/iterative-quick-sort/ |
| 10 | + * @author [Sebe324](https://github.com/sebe324) |
| 11 | + */ |
| 12 | + |
| 13 | +#include <iostream> /// for std::cout |
| 14 | +#include <vector> /// for std::vector |
| 15 | +#include <stack> /// for std::stack |
| 16 | +#include <algorithm> /// for std::is_sorted |
| 17 | +#include <cassert> /// for assert |
| 18 | + |
| 19 | + |
| 20 | +/** |
| 21 | + * @namespace sorting |
| 22 | + * @brief Sorting algorithms |
| 23 | + */ |
| 24 | +namespace sorting { |
| 25 | +/** |
| 26 | + * @brief The partition function sorts the array from |
| 27 | + * start to end and uses the last element as the pivot. |
| 28 | + * @param arr the array to be sorted |
| 29 | + * @param start starting index |
| 30 | + * @param end ending index |
| 31 | + * @return int next index of the pivot |
| 32 | + */ |
| 33 | +int partition(std::vector<int> &arr, int start, int end) |
| 34 | +{ |
| 35 | + int pivot = arr[end]; |
| 36 | + int index = start - 1; |
| 37 | + |
| 38 | + for (int j = start; j < end; j++) { |
| 39 | + if (arr[j] <= pivot) { |
| 40 | + std::swap(arr[++index], arr[j]); |
| 41 | + } |
| 42 | + } |
| 43 | + |
| 44 | + std::swap(arr[index + 1], arr[end]); |
| 45 | + return index + 1; |
| 46 | +} |
| 47 | + |
| 48 | +/** |
| 49 | + * @brief The main sorting function |
| 50 | + * @details The iterative quick sort uses |
| 51 | + * the stack instead of recursion for saving |
| 52 | + * and restoring the environment between calls. |
| 53 | + * It does not need the end and start params, because |
| 54 | + * it is not recursive. |
| 55 | + * @param arr array to be sorted |
| 56 | + * @return void |
| 57 | + */ |
| 58 | +void iterativeQuickSort(std::vector<int> &arr) |
| 59 | +{ |
| 60 | + std::stack<int> stack; |
| 61 | + int start = 0; |
| 62 | + int end = arr.size()-1; |
| 63 | + stack.push(start); |
| 64 | + stack.push(end); |
| 65 | + |
| 66 | + while(!stack.empty()) |
| 67 | + { |
| 68 | + end = stack.top(); |
| 69 | + stack.pop(); |
| 70 | + start = stack.top(); |
| 71 | + stack.pop(); |
| 72 | + |
| 73 | + int pivotIndex = partition(arr,start,end); |
| 74 | + |
| 75 | + if(pivotIndex -1 > start) |
| 76 | + { |
| 77 | + stack.push(start); |
| 78 | + stack.push(pivotIndex-1); |
| 79 | + } |
| 80 | + |
| 81 | + if(pivotIndex+1<end) |
| 82 | + { |
| 83 | + stack.push(pivotIndex+1); |
| 84 | + stack.push(end); |
| 85 | + } |
| 86 | + } |
| 87 | +} |
| 88 | + |
| 89 | +} // namespace sorting |
| 90 | +/** |
| 91 | + * @brief Self-test implementations |
| 92 | + * @returns void |
| 93 | + */ |
| 94 | +void tests() |
| 95 | +{ |
| 96 | + //TEST 1 - Positive numbers |
| 97 | + std::vector<int> case1={100,534,1000000,553,10,61,2000,238,2756,9,12,56,30}; |
| 98 | + std::cout<<"TEST 1\n"; |
| 99 | + std::cout<<"Before: \n"; |
| 100 | + for(auto x : case1) std::cout<<x<<","; |
| 101 | + std::cout<<"\n"; |
| 102 | + sorting::iterativeQuickSort(case1); |
| 103 | + assert(std::is_sorted(std::begin(case1),std::end(case1))); |
| 104 | + std::cout<<"Test 1 succesful!\n"; |
| 105 | + std::cout<<"After: \n"; |
| 106 | + for(auto x : case1) std::cout<<x<<","; |
| 107 | + std::cout<<"\n"; |
| 108 | + |
| 109 | + //TEST 2 - Negative numbers |
| 110 | + std::vector<int> case2={-10,-2,-5,-2,-3746,-785,-123, -452, -32456}; |
| 111 | + std::cout<<"TEST 2\n"; |
| 112 | + std::cout<<"Before: \n"; |
| 113 | + for(auto x : case2) std::cout<<x<<","; |
| 114 | + std::cout<<"\n"; |
| 115 | + sorting::iterativeQuickSort(case2); |
| 116 | + assert(std::is_sorted(std::begin(case2),std::end(case2))); |
| 117 | + std::cout<<"Test 2 succesful!\n"; |
| 118 | + std::cout<<"After: \n"; |
| 119 | + for(auto x : case2) std::cout<<x<<","; |
| 120 | + std::cout<<"\n"; |
| 121 | +} |
| 122 | + |
| 123 | + |
| 124 | +/** |
| 125 | + * @brief Main function |
| 126 | + * @returns 0 on exit |
| 127 | + */ |
| 128 | +int main() |
| 129 | +{ |
| 130 | + tests(); // run self test implementation |
| 131 | + return 0; |
| 132 | +} |
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