[Swift]LeetCode561. 数组拆分 I | Array Partition I

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Given an array of 2n integers, your task is to group these integers into n pairs of integer, say (a1, b1), (a2, b2), ..., (an, bn) which makes sum of min(ai, bi) for all i from 1 to n as large as possible.

Example 1:

Input: [1,4,3,2]

Output: 4
Explanation: n is 2, and the maximum sum of pairs is 4 = min(1, 2) + min(3, 4).

 Note:

1. n is a positive integer, which is in the range of [1, 10000].
2. All the integers in the array will be in the range of [-10000, 10000].

---

给定长度为 2n 的数组, 你的任务是将这些数分成 n 对, 例如 (a1, b1), (a2, b2), ..., (an, bn) ，使得从1 到 n 的 min(ai, bi) 总和最大。

示例 1:

输入: [1,4,3,2]

输出: 4
解释: n 等于 2, 最大总和为 4 = min(1, 2) + min(3, 4).

提示:

1. n 是正整数,范围在 [1, 10000].
2. 数组中的元素范围在 [-10000, 10000].

---

128ms

class Solution {
    func arrayPairSum(_ nums: [Int]) -> Int {
        //升序排序,使每一对的两个数字尽可能接近
        var arr:[Int] = nums.sorted(by: <)
        var sum:Int = 0
        //奇数位置(偶数索引)求和
        for i in stride(from: 0,to: arr.count,by: 2)
        {
            sum += arr[i]
        }
        return sum
    }
}

---

192ms

class Solution {
    func arrayPairSum(_ nums: [Int]) -> Int {
        var tempSum = 0
        let sortedArray = nums.sorted(by: <)
        var tempPair = [Int]()
        for number in sortedArray {
            if tempPair.count < 1 {
                tempPair.append(number)
            } else {
                if let min = tempPair.min() {
                    tempSum += min
                }
                tempPair.removeAll()
            }
        }
        return tempSum  
    }
}

---

196ms

class Solution {
    func arrayPairSum(_ nums: [Int]) -> Int {
        var temps = nums
        quickSort(numbers: &temps, start: 0, end: temps.count - 1)
        var sum = 0
        for (index, num) in temps.enumerated() {
            if index % 2 == 0 {
                sum = sum + num
            }
        }
        return sum
    }
    
    func quickSort(numbers: inout [Int], start: Int, end: Int) {
        if start > end {
            return
        }

        let pivot = partition(numbers: &numbers, start: start, end: end)

        quickSort(numbers: &numbers, start: start, end: pivot - 1)
        quickSort(numbers: &numbers, start: pivot + 1, end: end)
    }

    func partition(numbers: inout [Int], start: Int, end: Int) -> Int {
        let pivot = numbers[start]
        var left = start
        var right = end

        var index = start
        while left <= right  {
            while right >= left {
                if numbers[right] < pivot {
                    numbers[index] = numbers[right]
                    index = right
                    left = left + 1
                    break
                }
                right = right - 1
            }

            while right >= left {
                if numbers[left] > pivot {
                    numbers[index] = numbers[left]
                    index = left
                    right = right - 1
                    break
                }
                left = left + 1
            }
        }

        numbers[index] = pivot
        return index
    }
}