How can I avoid this integer overflow? - c++

Question

I was solving the famous leetcode question 4sum and I came up with an optimal solution but I am facing an integer overflow even though I have used long long int datatype.
class Solution {
public:
vector<vector<int>> fourSum(vector<int>& nums, int target) {
vector<vector<int>> res;
if(nums.empty()){
return res;
}
int n = nums.size();
sort(nums.begin(), nums.end());
for(int i=0; i<n; i++){
for(int j=i+1; j<n; j++){
long long int temp = target - (nums[j] + nums[i]);
int front = j+1;
int back = n-1;
while(front < back){
long long int twosum = nums[front] + nums[back];
if(twosum < temp){
front++;
}
else if(twosum > temp){
back--;
}
else{
vector<int> ans(4,0);
ans[0] = (nums[i]);
ans[1] = (nums[j]);
ans[2] = (nums[front]);
ans[3] = (nums[back]);
res.push_back(ans);
while(front < back && nums[front] == ans[2]) ++front;
while(front < back && nums[back] == ans[3]) --back;
}
}
while(j+1 < n && nums[j+1] == nums[j]) ++j;
}
while(i+1 < n && nums[i+1] == nums[i]) ++i;
}
return res;
}
};
Here is the error message:
Line 12: Char 41: runtime error: signed integer overflow: -294967296 - 2000000000 cannot be represented in type 'int' (solution.cpp)
SUMMARY: UndefinedBehaviorSanitizer: undefined-behavior prog_joined.cpp:21:41

Answer 1

Although in this compound statement
long long int temp = target - (nums[j] + nums[i]);
the destination variable temp is of sufficient size, the actual arithmetic happens on the types determined from integer promotion rules, applied to target and nums, both of which are of just int.
You'll have to upcast them to a sufficiently large type before the arithmetic operator takes effect. Like this
long long int temp = (long long int)target - ((long long int)nums[j] + (long long int)nums[i]);