303. Range Sum Query - Immutable (Easy)

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.

Solution: DP

class NumArray {
public:
    NumArray(vector<int> &nums) {
        dp.push_back(0);
        for (int i = 0; i < nums.size(); ++i) {
            dp.push_back(nums[i]+dp[i]);
        }
    }

    int sumRange(int i, int j) {
        return dp[j+1]-dp[i];
    }
private:
    vector<int> dp;
};


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