[leetcode] 303. Range Sum Query - Immutable

题目：

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.

代码：

class NumArray {
private:
    vector<int> mynums;
public:
    NumArray(vector<int> nums):mynums(nums){
    }
    
    int sumRange(int i, int j) {
        int sum = 0;
        for(int k = i; k <= j; k++){
            sum += this -> mynums[k];
        }
        return sum;
    }
};

/**
 * Your NumArray object will be instantiated and called as such:
 * NumArray obj = new NumArray(nums);
 * int param_1 = obj.sumRange(i,j);
 */

class NumArray {
public:
    NumArray(vector<int> nums) {
        presum.clear();
        presum.push_back(0);
        for (int num : nums) {
            presum.push_back(presum.back() + num);
        }
    }
    
    int sumRange(int i, int j) {
        return presum[j+1] - presum[i];
    }
    
    vector<int> presum;
};

/**
 * Your NumArray object will be instantiated and called as such:
 * NumArray obj = new NumArray(nums);
 * int param_1 = obj.sumRange(i,j);
 */