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:

1. You may assume that the array does not change.
2. 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]);  
    }  
};