leetcode-303-range sum query- immutable

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.

给定一个数组，多次查询从i到j的和，实现它。

利用一个数组存储从头到I的总和。那么i-j的和用从头到j和减去从头到i-1的和，即为所求。

class NumArray {
public:
    vector<int> dp;

    NumArray(vector<int> &nums) {
     dp.resize(nums.size());
     if(!nums.empty()){
     dp[0]=nums[0];
     for(int i=1;i<nums.size();i++)
     dp[i]=dp[i-1]+nums[i];
     }
    }

    int sumRange(int i, int j) {
        return i==0?dp[j]:dp[j]-dp[i-1];
    }
};


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