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class NumArray {
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 public:
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  vector<int> v; // vector to store input vector.
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  int sum;       // sum of all element of vector
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  NumArray(vector<int> &nums) {
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    v = nums;
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    sum = 0;
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    for (int i = 0; i < nums.size(); i++) {
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      sum += nums[i];
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    }
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  }
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  void update(int index, int val) {
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    sum -= v[index]; // subtract old element  from sum at index and then update
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                     // by adding new element val.
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    v[index] = val;
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    sum += val;
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  }
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  int sumRange(int left, int right) {
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    int res = sum;
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    for (int i = 0; i < left; i++) { // subtract all element before left and after right
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      res -= v[i];
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    }
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    for (int i = right + 1; i < v.size(); i++) {
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      res -= v[i];
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    }
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    return res; // return res ie. our answer.
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  }
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};