Leetcode - 303 area and retrieval - array immutable (design prefix and array)

class NumArray {
    
    private int[] preSum;
    public NumArray(int[] nums) {
    
       int n = nums.length;
       preSum = new int[n+1];
       for(int i = 0; i < n;i++)
         preSum[i+1] = preSum[i] + nums[i];
    }
    
    public int sumRange(int left, int right) {
    
         return preSum[right+1] - preSum[left];
    }
}