Topic
- Dynamic Programming
Description
https://leetcode.com/problems/range-sum-query-immutable/
Given an integer array nums, find the sum of the elements between indices i and j (i ≤ j), inclusive.
Implement the NumArray class:
NumArray(int[] nums)Initializes the object with the integer array nums.int sumRange(int i, int j)Return the sum of the elements of the nums array in the range[i, j]inclusive (i.e.,sum(nums[i], nums[i + 1], ... , nums[j]))
Example 1:
Input
["NumArray", "sumRange", "sumRange", "sumRange"]
[[[-2, 0, 3, -5, 2, -1]], [0, 2], [2, 5], [0, 5]]
Output
[null, 1, -1, -3]
Explanation
NumArray numArray = new NumArray([-2, 0, 3, -5, 2, -1]);
numArray.sumRange(0, 2); // return 1 ((-2) + 0 + 3)
numArray.sumRange(2, 5); // return -1 (3 + (-5) + 2 + (-1))
numArray.sumRange(0, 5); // return -3 ((-2) + 0 + 3 + (-5) + 2 + (-1))
Constraints:
- 0 <= nums.length <= 10⁴
- -10⁵ <= nums[i] <= 10⁵
- 0 <= i <= j < nums.length
- At most 10⁴ calls will be made to sumRange.
Analysis
略
Submission
//我写的
public class RangeSumQueryImmutable {
private int[] sums;
public RangeSumQueryImmutable(int[] nums) {
sums = new int[nums.length];
int sum = 0;
for (int i = 0; i < nums.length; i++) {
sum += nums[i];
sums[i] = sum;
}
}
public int sumRange(int i, int j) {
return sums[j] - (i == 0 ? 0 : sums[i - 1]);
}
}
// 别人写的
class RangeSumQueryImmutable2 {
private int[] sums;
public RangeSumQueryImmutable2(int[] nums) {
for (int i = 1; i < nums.length; i++)
nums[i] += nums[i - 1];
this.sums = nums;
}
public int sumRange(int i, int j) {
if (i == 0)
return sums[j];
return sums[j] - sums[i - 1];
}
}
Test
import static org.junit.Assert.*;
import org.junit.Test;
public class RangeSumQueryImmutableTest {
@Test
public void test() {
RangeSumQueryImmutable obj = new RangeSumQueryImmutable(new int[] {
-2, 0, 3, -5, 2, -1});
assertEquals(1, obj.sumRange(0, 2));
assertEquals(-1, obj.sumRange(2, 5));
assertEquals(-3, obj.sumRange(0, 5));
}
@Test
public void test2() {
RangeSumQueryImmutable2 obj = new RangeSumQueryImmutable2(new int[] {
-2, 0, 3, -5, 2, -1 });
assertEquals(1, obj.sumRange(0, 2));
assertEquals(-1, obj.sumRange(2, 5));
assertEquals(-3, obj.sumRange(0, 5));
}
}