LeetCode:Range Sum Query - Immutable

问题描述：

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.

思路：
1、由于多次调用，所以要将求和放在构造函数中实现；

2、在sumRange中只需将前面的和减去就可以。

JAVA代码：

public class NumArray {
int[] sums;

public NumArray(int[] nums) {
sums = new int[nums.length];
System.arraycopy(nums, 0, sums, 0, nums.length);
for(int i = 1; i<sums.length; i++) {
sums[i] += sums[i-1];
}
}

public int sumRange(int i, int j) {
if(i > j || i < 0 || j < 0 || j >= sums.length) return 0;
return i == 0 ? sums[j] : (sums[j] - sums[i-1]);
}
};