Range Sum Query - Immutable(easy)

1.这道题目与pat中的1046. Shortest Distance (20)相类似；

2.使用一个数组dp[i],记录0到第i个数的和

3.求i到j之间的和时，输出dp[j]-dp[i]+num[i]即可。

AC代码如下：

class NumArray {
public:

    vector<int> dp;
    vector<int> num;
    NumArray(vector<int> &nums) {
        int n=nums.size();
        dp=vector<int>(n,0);
        num=nums;
        for(int i=0;i<n;i++)
        {
            if(i>0)
                dp[i]=dp[i-1]+nums[i];
            else
                dp[0]=nums[0];
        }
    }

    int sumRange(int i, int j) {
        return dp[j]-dp[i]+num[i];
    }
};

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