[Swift]LeetCode303. 区域和检索 - 数组不可变 | Range Sum Query - Immutable

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Given an integer array nums, find the sum of the elements between indices i and j (i ≤ j), inclusive.

Example:

Given nums = [-2, 0, 3, -5, 2, -1]

sumRange(0, 2) -> 1
sumRange(2, 5) -> -1
sumRange(0, 5) -> -3 

Note:

1. You may assume that the array does not change.
2. There are many calls to sumRange function.

给定一个整数数组  nums，求出数组从索引 i 到 j  (i ≤ j) 范围内元素的总和，包含 i,  j 两点。

示例：

给定 nums = [-2, 0, 3, -5, 2, -1]，求和函数为 sumRange()

sumRange(0, 2) -> 1
sumRange(2, 5) -> -1
sumRange(0, 5) -> -3

说明:

1. 你可以假设数组不可变。
2. 会多次调用 sumRange 方法。

---

 132ms

class NumArray {
    let sum: [Int]
    init(_ nums: [Int]) {
        var s = 0
        var sum = [0]
        for n in nums {
            s += n
            sum.append(s)
        }
        self.sum = sum
    }

    func sumRange(_ i: Int, _ j: Int) -> Int {
        return sum[j + 1] - sum[i]
    }
}

---

132ms

class NumArray {
    
    let _nums : [Int]
    var sums : [Int]
    
    init(_ nums: [Int]) {
        _nums = nums
        
        sums = [Int]()
        if nums.isEmpty {
            return
        }
        sums.append(_nums[0])
        for i in 1..<nums.count {
            sums.append(sums[i-1] + _nums[i])
        }
        
    }
    
    @inline(__always) func sumRange(_ i: Int, _ j: Int) -> Int {
        if i == 0 {
            return sums[j]
        }
        return sums[j]-sums[i-1]
    }
}

/**
 * Your NumArray object will be instantiated and called as such:
 * let obj = NumArray(nums)
 * let ret_1: Int = obj.sumRange(i, j)
 */
 

---

172ms

class NumArray {
    
    private var sums: [Int] = []

    init(_ nums: [Int]) {
        guard !nums.isEmpty else {
            return
        }
        
        var sums = Array(repeating: 0, count: nums.count)
        sums[0] = nums[0]
        for index in 1..<nums.count {
            let num = nums[index]
            sums[index] = sums[index - 1] + num
        }
    
        self.sums = sums
    }
    
    func sumRange(_ i: Int, _ j: Int) -> Int {
        if i == 0 {
            return sums[j]
        } else {
            return sums[j] - sums[i - 1]
        }
    }
}

---

2444ms

class NumArray {
    let nums: [Int]
    
    init(_ nums: [Int]) {
        self.nums = nums
    }
    
    func sumRange(_ i: Int, _ j: Int) -> Int
    {
        var sum = 0
        for index in i...j
        {
            sum += nums[index]
        }
        return sum
    }
}

---

3328ms

class NumArray {

    var nums: [Int]
    
    init(_ nums: [Int]) {
        self.nums = nums
    }
    
    func sumRange(_ i: Int, _ j: Int) -> Int {
        
        guard i <= j && i < nums.count else {
            return 0
        }
        
        let i = i < 0 ? 0 : i
        let j = j < nums.count ? j : nums.count-1
        
        var sum = 0
        for k in i...j {
            sum += nums[k]
        }

        return sum
    }
}