1. /**
    2.  * @param {number[]} nums
    3.  */
    4. var NumArray = function(nums) {
    5.     this.preSum = [0]
    7.     for(let i = 1; i <= nums.length; i++) {
    8.         this.preSum[i] = this.preSum[i - 1] + nums[i - 1]
    9.     }
    10. };
    12. /** 
    13.  * @param {number} left 
    14.  * @param {number} right
    15.  * @return {number}
    16.  */
    17. NumArray.prototype.sumRange = function(left, right) {
    19.     // 前缀和， sumRange(i, j) = preSum[j] - preSum[i - 1]
    20.     return this.preSum[right + 1] - this.preSum[left]
    21. };
    23. /**
    24.  * Your NumArray object will be instantiated and called as such:
    25.  * var obj = new NumArray(nums)
    26.  * var param_1 = obj.sumRange(left,right)
    27.  */