leetcode303-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.

问题求解：

class NumArray {
vector<int> sum;
public:
NumArray(vector<int> &nums) {
if(nums.size()==0) return;
sum = vector<int> (nums.size(), 0);//vector使用前要初始化
sum[0]=nums[0];
for(int i=1;i<nums.size();i++)
{
sum[i] = sum[i-1]+nums[i];//状态转移方程
}
}

int sumRange(int i, int j) {
return (i==0)?sum[j]:sum[j]-sum[i-1];//下标i~j之间数的和
}
};

// Your NumArray object will be instantiated and called as such:
// NumArray numArray(nums);
// numArray.sumRange(0, 1);
// numArray.sumRange(1, 2);