JS 【算法】二分查找

const arr = [1, 2, 3, 4, 5, 6, 7, 8, 9]
const target = 2
console.log(binarySearch1(arr, target))
console.log(binarySearch2(arr, target))

        function binarySearch1(arr, target) {
            const length = arr.length
            if (length === 0) return -1

            let startIndex = 0 // 开始位置
            let endIndex = length - 1 // 结束位置

            while (startIndex <= endIndex) {
                const midIndex = Math.floor((startIndex + endIndex) / 2) // 中间位置
                const midValue = arr[midIndex]
                if (target < midValue) {
                    // 目标值较小，则继续在左侧查找
                    endIndex = midIndex - 1
                } else if (target > midValue) {
                    // 目标值较大，则继续在右侧查找
                    startIndex = midIndex + 1
                } else {
                    // 相等，则返回
                    return midIndex
                }
            }

            return -1 // 未找到
        }

        function binarySearch2(arr, target, startIndex, endIndex) {
            const length = arr.length
            if (length === 0) return -1

            // 开始和结束的范围
            if (startIndex == null) startIndex = 0
            if (endIndex == null) endIndex = length - 1

            // 开始和结束相遇，则结束
            if (startIndex === endIndex) return -1

            // 中间位置
            const midIndex = Math.floor((startIndex + endIndex) / 2)
            const midValue = arr[midIndex]

            if (target < midValue) {
                // 目标值较小，则从左侧寻找
                return binarySearch2(arr, target, startIndex, midIndex - 1)
            } else if (target > midValue) {
                // 目标值较大，则从右侧查找
                return binarySearch2(arr, target, midIndex + 1, endIndex)
            } else {
                // 相等，则返回
                return midIndex
            }
        }