[Swift]LeetCode238. 除自身以外数组的乘积 | Product of Array Except Self

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Given an array nums of n integers where n > 1,  return an array outputsuch that output[i] is equal to the product of all the elements of numsexcept nums[i].

Example:

Input:  [1,2,3,4]
Output: [24,12,8,6]

Note: Please solve it without division and in O(n).

Follow up:
Could you solve it with constant space complexity? (The output array does not count as extra space for the purpose of space complexity analysis.)

---

给定长度为 n 的整数数组 nums，其中 n > 1，返回输出数组 output ，其中 output[i] 等于 nums 中除 nums[i] 之外其余各元素的乘积。

示例:

输入: [1,2,3,4]
输出: [24,12,8,6]

说明: 请不要使用除法，且在 O(n) 时间复杂度内完成此题。

进阶：
你可以在常数空间复杂度内完成这个题目吗？（ 出于对空间复杂度分析的目的，输出数组不被视为额外空间。）

---

116ms

class Solution {
    func productExceptSelf(_ nums: [Int]) -> [Int] {

    var result : [Int] = []
    var mutiplier = 1

    for num in nums
    {
        result.append(mutiplier)
        mutiplier *= num
    }

    mutiplier = 1
    var index = nums.count - 1
    while index>=0
    {
        result[index] *= mutiplier
        mutiplier *= nums[index]
        index -= 1
    }

    return result
}
}

---

124ms

class Solution {
    func productExceptSelf(_ nums: [Int]) -> [Int] {
        guard !nums.isEmpty else { return [] }

        var output = [Int](repeating: 1, count: nums.count)
        for i in nums.indices.dropFirst() {
            output[i] = output[i - 1] * nums[i - 1]
        }
        
        var right = nums[nums.count - 1]
        for i in nums.indices.dropLast().reversed() {
            output[i] *= right
            right *= nums[i]
        }
        
        return output
    }
}

---

124ms

class Solution {
    func productExceptSelf(_ nums: [Int]) -> [Int] {
        let count = nums.count
        if count == 0 {
            return [Int]()
        }
        if count == 1 {
            return [1]
        }
        var result = nums
        for i in 1..<count - 1 {
            result[i] *= result[i - 1]
        }
        
        var temp = 1
        for i in (1..<count).reversed() {
            result[i] = temp * result[i - 1]
            temp *= nums[i]
        }
        result[0] = temp
        return result
    }
}

---

125ms

class Solution {
    func productExceptSelf(_ nums: [Int]) -> [Int] {
        let total = nums.filter({ $0 != 0 }).reduce(1, { $0 * $1 })
        let zero_count = nums.filter({ $0 == 0}).count
        return nums.map { number -> Int in
            if zero_count == 0 {
                return total / number
            } else if zero_count == 1 {
                return number == 0 ? total : 0
            } else {
                return 0
            }
        }
    }
}

---

136ms

class Solution {
    func productExceptSelf(_ nums: [Int]) -> [Int] {
        guard nums.count > 1 else { return [0]}
        let letfProduct = nums.reduce(into:[]) { $0.append(($0.last ?? 1) * $1) }
        let rightProduct = Array(Array(nums.reversed().reduce(into:[]) { $0.append(($0.last ?? 1) * $1) }).reversed())
        var result = [Int](repeating:0, count:nums.count)
        for i in 0..<nums.count {
            if i == 0 {
                result[i] = rightProduct[1]
            } else if i == nums.count-1 {
                result[i] = letfProduct[nums.count-2]
            } else {
                result[i] = letfProduct[i-1] * rightProduct[i+1]
            }
        }
        return result
    }
}

---

160ms

class Solution {
    func productExceptSelf(_ nums: [Int]) -> [Int] {
        let n = nums.count
        var result = [Int](repeating: 1, count: n)
        guard n > 0 else { return result }
        //设left[i]为nums[i]左边的所有元素的乘积
        var left = [Int](repeating: 1, count: n) 
        //设right[i]为nums[i]右边的所有元素的乘积
        var right = [Int](repeating: 1, count: n) 
        for i in 1 ..< n {
            left[i] = left[i - 1] * nums[i - 1]
        }
        for i in (0 ..< n - 1).reversed() {
            right[i] = right[i + 1] * nums[i + 1]
        }
        for i in 0 ..< n {
            result[i] = left[i] * right[i]
        }
        return result
    }
}

---

188ms

class Solution {
    func productExceptSelf(_ nums: [Int]) -> [Int] {
        var res = [Int]()
        var leftValue = 1
        for i in 0 ..< nums.count {
            res.append(leftValue)
            leftValue *= nums[i]
        }
        var rightValue = 1
        for i in stride(from: nums.count - 1, to: -1, by: -1) {
            res[i] = res[i] * rightValue
            rightValue *= nums[i]
        }
        return res
    }
}