We can find the missing number by subtracting sum of first n positive integer (sumAll) with sum of all number in nums (sumNum). sumAll can be easily computed using Triangular Number.

```
int missingNumber(int* nums, int numsSize) {
int sumAll = (numsSize * (numsSize + 1)) / 2;
int sumNum = 0;
for (int i = 0; i < numsSize; i++) {
sumNum += nums[i];
}
return (sumAll - sumNum);
}
```