ts-ds-tool/resource/algorithms/randomized-select/randomizedSelect.js

Data structure and algorithm of TypeScript

import { defaultCompare, random, swap } from "../../util";
export function randomizedSelect(arr, i, compare = defaultCompare) {
    if (!arr || !arr.length || i <= 0 || i > arr.length) {
        return null;
    }
    const tempArr = Array.prototype.slice.call(arr);
    return select(tempArr, 0, tempArr.length - 1, i);
    function select(array, start, end, num) {
        if (start === end) {
            return array[start];
        }
        const index = random(start, end);
        swap(array, start, index);
        const middle = partition(array, start, end);
        const lowNum = middle - start + 1;
        if (num === lowNum) {
            return array[middle];
        }
        else if (num < lowNum) {
            return select(array, start, middle - 1, num);
        }
        else {
            return select(array, middle + 1, end, num - lowNum);
        }
    }
    function partition(array, p, r) {
        const x = array[p];
        let currentIndex = p;
        for (let j = p + 1; j <= r; j++) {
            if (compare(x, array[j])) {
                currentIndex++;
                swap(array, currentIndex, j);
            }
        }
        swap(array, currentIndex, p);
        return currentIndex;
    }
}