[Swift]LeetCode528. 按权重随机选择 | Random Pick with Weight

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Given an array w of positive integers, where w[i] describes the weight of index i, write a function pickIndex which randomly picks an index in proportion to its weight.

Note:

1. 1 <= w.length <= 10000
2. 1 <= w[i] <= 10^5
3. pickIndex will be called at most 10000 times.

Example 1:

Input: 
["Solution","pickIndex"]
[[[1]],[]]
Output: [null,0]

Example 2:

Input: 
["Solution","pickIndex","pickIndex","pickIndex","pickIndex","pickIndex"]
[[[1,3]],[],[],[],[],[]]
Output: [null,0,1,1,1,0]

Explanation of Input Syntax:

The input is two lists: the subroutines called and their arguments. Solution's constructor has one argument, the array w. pickIndex has no arguments. Arguments are always wrapped with a list, even if there aren't any.

---

给定一个正整数数组 w ，其中 w[i] 代表位置 i 的权重，请写一个函数 pickIndex ，它可以随机地获取位置 i，选取位置 i 的概率与 w[i] 成正比。

说明:

1. 1 <= w.length <= 10000
2. 1 <= w[i] <= 10^5
3. pickIndex 将被调用不超过 10000 次

示例1:

输入: 
["Solution","pickIndex"]
[[[1]],[]]
输出: [null,0]

示例2:

输入: 
["Solution","pickIndex","pickIndex","pickIndex","pickIndex","pickIndex"]
[[[1,3]],[],[],[],[],[]]
输出: [null,0,1,1,1,0]

输入语法说明：

输入是两个列表：调用成员函数名和调用的参数。Solution 的构造函数有一个参数，即数组 w。pickIndex 没有参数。输入参数是一个列表，即使参数为空，也会输入一个 [] 空列表。

---

704ms

class Solution {
    var total: Int
    var pTotal: [Int]
    init(_ w: [Int]) {
        total = 0
        pTotal = [Int]()
        for x in w{
            total += x
            pTotal.append(total)
        }
    }
    
    func pickIndex() -> Int {
        let index = Int.random(in: 0..<total)
        var lo = 0, hi = pTotal.count-1
        while lo != hi{
            let mid = (lo+hi)/2
            if pTotal[mid] <= index{
                lo = mid + 1
            }else{
                hi = mid
            }
        }
        return lo
    }
}

/**
 * Your Solution object will be instantiated and called as such:
 * let obj = Solution(w)
 * let ret_1: Int = obj.pickIndex()
 */

---

720ms

class Solution {

    let nums:[Int]
    
    init(_ w: [Int]) {
        var sum = 0
        var result = [Int]()
        w.forEach {
            sum += $0
            result.append(sum)
        }
        nums = result
    }
    
    func pickIndex() -> Int {
       let sum = nums.last!
       let num = Int.random(in: 0..<sum) % sum
        var left = 0
        var right = nums.count - 1
        while left + 1 < right {
            let mid = left + (right - left) / 2
            if nums[mid] > num {
                right = mid
            } else {
                left = mid
            }
        }
        
        if nums[left] > num {
            return left
        } else {
            return right
        }
    }
}

---

752ms

class Solution {
    
    let w: [Int]
    let sum: Int

    init(_ w: [Int]) {
        sum = w.reduce(0, +)
        self.w = w.reduce([Int]()){ 
            let n = ($0.last ?? 0) + $1
            return $0 + [n]
        }
    }
    
    func pickIndex() -> Int {
        let random = Int.random(in: 0...sum-1)
        return searchInsert(w, random)
    }
    
    func searchInsert(_ nums: [Int], _ target: Int) -> Int {
        var low = 0
        var high = nums.count-1
        while low <= high {
            let middle = (low + high) / 2
            if nums[middle] == target {
                return middle+1
            } else if nums[middle] > target {
                high = middle - 1
            } else {
                low = middle + 1
            }
        }
        return low
    }
}

---

772ms

class Solution {
    
    var sum: [Int]

    init(_ w: [Int]) {
        sum = [Int]()
        sum.append(w[0])
        for i in 1..<w.count {
            sum.append(sum[i-1] + w[i])
        }
    }
    
    func pickIndex() -> Int {
        let value = Int.random(in: 1...sum.last!)
        return bSearch(value, 0, sum.count - 1)
    }
    
    private func bSearch(_ value: Int, _ start: Int, _ end: Int) -> Int {
        let mid = start + (end - start) / 2
        if sum[mid] >= value && (start == mid || sum[mid - 1] < value) { return mid }
        if sum[mid] < value {
            return bSearch(value, mid + 1, end)
        }
        return bSearch(value, start, mid - 1)
    }
}

---

784ms

class Solution {

    private let accSum: [Int]

    init(_ w: [Int]) {
        self.accSum = w.reduce([]) { $0 + [($0.last ?? 0) + $1]}
    }
    
    func pickIndex() -> Int {
        let num = Int.random(in: 1 ... accSum.last!)  
        var (left, right) = (0, accSum.count - 1)
        while left < right {
            let mid = left + (right - left) / 2
            if accSum[mid] == num {
                return mid
            } else if accSum[mid] < num {
                left = mid + 1
            } else {
                right = mid
            }
        }

        return left
    }
}

---

Runtime: 976 ms

Memory Usage: 19.9 MB

class Solution {
    var sum:[Int]

    init(_ w: [Int]) {
        sum = w
        for i in 1..<w.count
        {
            sum[i] += sum[i - 1]
        }        
    }
    
    func pickIndex() -> Int {
        var x:Int = Int.random(in:0..<sum.last!)
        var left:Int = 0
        var right:Int = sum.count - 1  
        var mid:Int = 0
        while (left < right)
        {
            mid = left + (right - left) / 2
            if sum[mid] <= x
            {
                left = mid + 1
            }
            else 
            {
                right = mid
            }            
        }        
        return right   
    }
}

/**
 * Your Solution object will be instantiated and called as such:
 * let obj = Solution(w)
 * let ret_1: Int = obj.pickIndex()
 */