算法-二分查找

递归实现

const list = [11, 12, 13, 14, 17]
function get12(arr, start, end, num) {
    console.log(arr, start, end, num)
    const mid = Math.floor((start + end) / 2) // Math.floor
    if (start > end) {
        return null
    }
    if (arr[mid] === num) {
        return mid
    }
    if (arr[mid] < num) {
        return get12(arr, mid + 1, end, num) // mid + 1
    }
    if (arr[mid] > num) {
        return get12(arr, start, mid - 1, num)
    }
}
console.log(get12(list, 0, list.length - 1, 17))

while实现

const list = [11, 12, 13, 18, 19]
function get12(arr, num) {
    let start = 0
    let end = arr.length - 1
    while(start <= end) {
        const mid = Math.floor((end + start) / 2) // Math.floor
        if (arr[mid] === num) {
            return mid
        }
        if (arr[mid] < num) {
            start = mid + 1 // mid + 1
        }
        if (arr[mid] > num) {
            end = mid - 1 // mid - 1
        }
    }
    return null
}
const l = console.log
l(get12(list, 19))