84 lines
2.1 KiB
C++
84 lines
2.1 KiB
C++
// CPP program for implementation of "Quick-select" method based on Quick-sort or partition algorithm
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//The Question here is to find the kth smallest element in an array.
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//if array = {10 20 30 40 50 60} and k=2 then k'th smallest element will be : 20
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#include <bits/stdc++.h>
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using namespace std;
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// Standard partition process of QuickSort().
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// It considers the last element as pivot
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// and moves all smaller element to left of
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// it and greater elements to right
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int partition(int arr[], int left, int right)
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{
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int x = arr[right], i = left;
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for (int j = left; j <= right - 1; j++)
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{
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if (arr[j] <= x)
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{
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swap(arr[i], arr[j]);
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i++;
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}
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}
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swap(arr[i], arr[right]);
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return i;
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}
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// This function returns k'th smallest
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// element in arr[l..r] using QuickSort
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// based method. ASSUMPTION: ALL ELEMENTS
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// IN ARR[] ARE DISTINCT
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int kthSmallest(int arr[], int left, int right, int k)
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{
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// If k is smaller than number of
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// elements in array
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if (k > 0 && k <= right - left + 1)
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{
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// Partition the array around last
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// element and get position of pivot
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// element in sorted array
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int index = partition(arr, left, right);
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// If position is same as k
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if (index - left == k - 1)
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return arr[index];
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// If position is more, recur
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// for left subarray
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if (index - left > k - 1)
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return kthSmallest(arr, left, index - 1, k);
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// Else recur for right subarray
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return kthSmallest(arr, index + 1, right,
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k - index + left - 1);
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}
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// If k is more than number of
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// elements in array
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return INT_MAX;
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}
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// Driver program to test above methods
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int main()
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{
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int size;
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cout << "Enter the size" << endl;
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cin >> size;
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int arr[size];
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cout << "Enter the array" << endl;
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for (int i = 0; i < size; i++)
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{
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cin >> arr[i];
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}
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int k;
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cout << "Enter the value of k (must be less than size)" << endl;
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cin >> k;
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cout << "K-th smallest element is "
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<< kthSmallest(arr, 0, size - 1, k);
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return 0;
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}
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