LeetCode: 303. Range Sum Query - Immutable

题目描述

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.

解题思路 —— 前缀和

记录下前缀和， 即，​​PrefixSum[i]​​​ 表示前 ​​i​​ 个数字的和。

特别地，当 ​​i = 0​​​ 时， ​​PrefixSum[i] = 0​​。

则 ​​sumRange(i, j) = PrefixSum[j] - PrefixSum[i]​​.

AC 代码

type NumArray struct {
    PrefixSum []int // 前缀和
}


func Constructor(nums []int) NumArray {
    arr := NumArray{ []int{0} }
    
    for i := 0; i < len(nums); i++{
        arr.PrefixSum = append(arr.PrefixSum, nums[i] + arr.PrefixSum[i])
    }
    
    return arr
}


func (this *NumArray) SumRange(i int, j int) int {
    return this.PrefixSum[j+1] - this.PrefixSum[i]
}


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