2 ms Java solution with explaination, O(n) init, O(1) call

  • 1

    sum[i] is the sum before i. (0~i-1). If we want to sum range (i~j), just sum[j+1] - sum[i] .

    public class NumArray {
        int[] sum;
        public NumArray(int[] nums) {
            final int N = nums.length;
            sum = new int[N+1];
            for(int i=1; i<=N; i++){
                sum[i] = sum[i-1] + nums[i-1];
        public int sumRange(int i, int j) {
            return sum[j+1] - sum[i];

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