1236. Algorithm - Pre Sum - DraftPre sum
Presum for multiple queries.
1. Range Sum Query - Immutable
int[] preSum;
public NumArray(int[] nums) {
if (nums == null || nums.length == 0) {
return;
}
preSum = new int[nums.length];
preSum[0] = nums[0];
for (int i = 1; i < nums.length; i++) {
preSum[i] = preSum[i - 1] + nums[i];
}
}
public int sumRange(int i, int j) {
if (i == 0) {
return preSum[j];
} else {
return preSum[j] - preSum[i - 1];
}
}