LeetCode之Range Sum Query - Immutable（Kotlin）

问题： 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.
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方法： 由于数组是不可变的，因此能够先建立和集，和集每一位存储数组前n项和，这样i到j的和就是前j项和减去前i项和，不须要每次都去循环获取i到j项的和，提升算法的效率，算法复杂度是O(1)。

具体实现：

class RangeSumQueryImmutable(nums: IntArray) {
    val sums = IntArray(nums.size + 1)

    init {
        for (index in nums.indices) {
            sums[index + 1] = sums[index] + nums[index]
        }
    }

    /**
     * Your NumArray object will be instantiated and called as such:
     * var obj = NumArray(nums)
     * var param_1 = obj.sumRange(i,j)
     */

    fun sumRange(i: Int, j: Int): Int {
        return sums[j + 1] - sums[i]
    }
}

fun main(args: Array<String>) {
    val rangeSumQueryImmutable = RangeSumQueryImmutable(intArrayOf(-2, 0, 3, -5, 2, -1))
    val sum = rangeSumQueryImmutable.sumRange(0, 2)
    println(sum)
}
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具体代码实现能够参考Github