Q307.cpp

class NumArray {
public:
    NumArray(vector<int> &_nums) {
        if(_nums.empty())
            return;
        nums=_nums;
        numSum.resize(nums.size(), 0);
        numSum[0]=nums[0];
        
        for(int i=1; i<nums.size(); i++)
            numSum[i] = nums[i]+numSum[i-1];
    }

    void update(int i, int val) {
        int diff = val - nums[i];
        nums[i]=val;
        for(; i<nums.size(); i++)
            numSum[i]=numSum[i]+diff;
    }

    int sumRange(int i, int j) {
        if(j<i)
            return 0;
        if(i==0)
            return numSum[j];
        else{
            return numSum[j]-numSum[i-1];
        }
    }
    vector<int> numSum;
    vector<int> nums;
};


// Your NumArray object will be instantiated and called as such:
// NumArray numArray(nums);
// numArray.sumRange(0, 1);
// numArray.update(1, 10);
// numArray.sumRange(1, 2);