JS求解topK--快排、大小堆顶

快排算法：

let quickSort = function (arr) {
    if (arr.length < 2) return arr
    let midValue = arr.splice(0, 1), left = [], right = []
    arr.forEach(el => {
        el > midValue ? right.push(el) : left.push(el)
    });
    return quickSort(left).concat(midValue, quickSort(right))
}
console.log(quickSort([6, 0, 1, 4, 9, 7, -3, 1, -4, -8, 4, -7, -3, 3, 2, -3, 9, 5, -4, 0]))
////[ -8, -7, -4, -4, -3, -3, -3, 0, 0, 1, 1, 2, 3, 4, 4, 5, 6, 7, 9, 9 ]

改造快排求topk：

let quickTopK = function (arr, k) {
    if(k==0)return []
    if (arr.length < 2) return arr
    let midValue = arr.splice(0, 1), left = [], right = []
    arr.forEach((el) => {
        el > midValue ? left.push(el) : right.push(el)
    });
    if (left.length == k) {
        return left
    } else if (left.length > k) {
        return quickTopK(left, k)
    } else {
        return left.concat(midValue, quickTopK(right, k - left.length - 1))
    }
}
console.log(quickTopK([6, 0, 1, 4, 9, 7, -3, 1, -4, -8, 4, -7, -3, 3, 2, -3, 9, 5, -4, 0], 8))
////[ 9, 7, 9, 6, 5, 4, 4, 3 ]

堆排序：

function swap(A, i, j) {
    let temp = A[i]
    A[i] = A[j]
    A[j] = temp
}

function shiftDown(A, i, length) {
    let temp
    for (let j = 2 * i + 1; j < length; j = 2 * j + 1) {
        temp = A[i]
        if (j + 1 < length && A[j] > A[j + 1]) {
            j++
        }
        if (temp > A[j]) {
            swap(A, i, j)
            i = j
        } else {
            break
        }
    }
}

function heapSort(A) {
    for (let i = Math.floor(A.length / 2 - 1); i >= 0; i--) {
        shiftDown(A, i, A.length)
    }
    for (let i = Math.floor(A.length - 1); i > 0; i--) {
        swap(A, 0, i)
        shiftDown(A, 0, i)
    }
    return A
}

小顶堆topK：

function swap(A, i, j) {
    let temp = A[i]
    A[i] = A[j]
    A[j] = temp
}

function shiftDown(A, i, length) {
    let temp
    for (let j = 2 * i + 1; j < length; j = 2 * j + 1) {
        temp = A[i]
        if (j + 1 < length && A[j] > A[j + 1]) {
            j++
        }
        if (temp > A[j]) {
            swap(A, i, j)
            i = j
        } else {
            break
        }
    }
}
function heapTopK(arr, k) {
    let A = arr.splice(0, k)
    for (let i = Math.floor(A.length / 2 - 1); i >= 0; i--) {
        shiftDown(A, i, A.length)
    }
    for (let i = 0; i < arr.length; i++) {
        if (arr[i] > A[0]) {
            A[0] = arr[i]
            for (let i = Math.floor(A.length / 2 - 1); i >= 0; i--) {
                shiftDown(A, i, A.length)
            }
        }
    }
    return A
}