68 lines
1.6 KiB
JavaScript
68 lines
1.6 KiB
JavaScript
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(function (exports) {
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'use strict';
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/**
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* Returns the n-th smallest element of list within
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* lo..hi inclusive (i.e. lo <= n <= hi).<br><br>
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* Time complexity: O(N).
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*
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* @example
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*
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* var quickselect = require('path-to-algorithms/src/searching/'+
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* 'quickselect').quickselect;
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* var result = quickselect([5, 1, 2, 2, 0, 3], 1, 0, 5);
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* console.log(result); // 1
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*
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* @public
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* @module searching/quickselect
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* @param {Array} arr Input array.
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* @param {Number} n A number of an element.
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* @param {Number} lo Low index.
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* @param {Number} hi High index.
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* @return Returns n-th smallest element.
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*/
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function quickselect(arr, n, lo, hi) {
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function partition(arr, lo, hi, pivotIdx) {
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function swap(arr, i, j) {
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var temp = arr[i];
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arr[i] = arr[j];
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arr[j] = temp;
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}
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var pivot = arr[pivotIdx];
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swap(arr, pivotIdx, hi);
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for (var i = lo; i < hi; i += 1) {
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if (arr[i] < pivot) {
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swap(arr, i, lo);
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lo += 1;
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}
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}
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swap(arr, hi, lo);
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return lo;
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}
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if (arr.length <= n) {
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return undefined;
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}
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lo = lo || 0;
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hi = hi || arr.length - 1;
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if (lo === hi) {
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return arr[lo];
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}
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while (hi >= lo) {
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var pivotIdx =
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partition(arr, lo, hi, lo + Math.floor(Math.random() * (hi - lo + 1)));
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if (n === pivotIdx) {
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return arr[pivotIdx];
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}
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if (n < pivotIdx) {
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hi = pivotIdx - 1;
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} else {
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lo = pivotIdx + 1;
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}
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}
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return undefined;
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}
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exports.quickselect = quickselect;
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})(typeof window === 'undefined' ? module.exports : window);
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