Question
Formatted question description: https://leetcode.ca/all/303.html
303 Range Sum Query - Immutable
Given an integer array nums, find the sum of the elements between indices i and j (i ≤ j), inclusive.
Example:
Given nums = [-2, 0, 3, -5, 2, -1]
sumRange(0, 2) -> 1
sumRange(2, 5) -> -1
sumRange(0, 5) -> -3
Note:
You may assume that the array does not change.
There are many calls to sumRange function.
@tag-array
Algorithm
Establish a cumulative sum array dp, where dp[i] represents the sum of the numbers in the interval [0, i], then [i,j] can be expressed as dp[j]-dp[i-1]
, pay attention here When i=0, just return dp[j] directly.
Code
Java
public class Range_Sum_Query_Immutable {
class NumArray {
private int[] sum;
public NumArray(int[] nums) {
sum = new int[nums.length + 1];
for (int i = 0; i < nums.length; i++) {
// sum x meaning sumup until index x-1
sum[i + 1] = sum[i] + nums[i];
}
}
public int sumRange(int i, int j) {
return sum[j + 1] - sum[i];
}
}
/**
* Your NumArray object will be instantiated and called as such:
* NumArray obj = new NumArray(nums);
* int param_1 = obj.sumRange(i,j);
*/
}