hello-algo/codes/swift/chapter_divide_and_conquer/binary_search_recur.swift

/**
 * File: binary_search_recur.swift
 * Created Time: 2023-09-02
 * Author: nuomi1 (nuomi1@qq.com)
 */

/* 二分查找：问题 f(i, j) */
func dfs(nums: [Int], target: Int, i: Int, j: Int) -> Int {
    // 若区间为空，代表无目标元素，则返回 -1
    if i > j {
        return -1
    }
    // 计算中点索引 m
    let m = (i + j) / 2
    if nums[m] < target {
        // 递归子问题 f(m+1, j)
        return dfs(nums: nums, target: target, i: m + 1, j: j)
    } else if nums[m] > target {
        // 递归子问题 f(i, m-1)
        return dfs(nums: nums, target: target, i: i, j: m - 1)
    } else {
        // 找到目标元素，返回其索引
        return m
    }
}

/* 二分查找 */
func binarySearch(nums: [Int], target: Int) -> Int {
    let n = nums.count
    // 求解问题 f(0, n-1)
    return dfs(nums: nums, target: target, i: 0, j: n - 1)
}

@main
enum BinarySearchRecur {
    /* Driver Code */
    static func main() {
        let target = 6
        let nums = [1, 3, 6, 8, 12, 15, 23, 26, 31, 35]

        // 二分查找（双闭区间）
        let index = binarySearch(nums: nums, target: target)
        print("目标元素 6 的索引 = \(index)")
    }
}