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public class NumArray {
    
    private int[] prefixSum;
    public NumArray(int[] nums) {
        int len = nums.length;
        prefixSum = new int[len + 1];
        
        for (int i = 1; i <= len; i++) {
            prefixSum[i] = prefixSum[i - 1] + nums[i - 1];
        }
    }
    
    public int sumRange(int i, int j) {
        return prefixSum[j + 1] - prefixSum[i];
    }
}

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