[LeetCode][JavaScript]Range Sum Query - Immutable

Range Sum Query - Immutable

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.

https://leetcode.com/problems/range-sum-query-immutable/

 

 

 

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求和，给定数组不会变，求和方法会反复调用多次。

维护一个数组，第n个值就是从0到n的和。

比如求2到5的和，就是0到5的和减去0到1的和。

 

/**
 * @constructor
 * @param {number[]} nums
 */
var NumArray = function(nums) {
    this.dict = [];
    var sum = 0;
    for(var i = 0; i < nums.length; i++){
        sum += nums[i];
        this.dict[i] = sum;
    }
};

/**
 * @param {number} i
 * @param {number} j
 * @return {number}
 */
NumArray.prototype.sumRange = function(i, j) {
    if(i === 0){
        return this.dict[j];
    }else{
        return this.dict[j] - this.dict[i - 1];
    }
};