Leetcode 303: 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.

 

public class NumArray {
    private int[] presum = null;
    
    public NumArray(int[] nums) {
        this.presum = new int[nums.Length];
        
        int pre = 0;
        
        for (int i = 0; i < presum.Length; i++)
        {
            presum[i] = pre + nums[i];
            pre = presum[i];
        }
    }
    
    public int SumRange(int i, int j) {
        return presum[j] - (i == 0 ? 0 : presum[i - 1]);
    }
}

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