###### tags: `learning` `zxm`
# Ask zxm: LeetCode #371 Sum of Two Integers
https://leetcode.com/problems/sum-of-two-integers/
## Success

int getSum(int a, int b){
    int carry; // 64-bit
    while (b != 0) {
        carry = a & b;
        a = a ^ b;
        b = (carry & 0xFFFFFFFF) << 1;  // limited to 32 bits
    }
    return a;
}

## Failure

int getSum(int a, int b){
    int carry; // 64-bit
    while (b != 0) {
        carry = a & b;
        a = a ^ b;
        b = (carry << 1);
    }
    return a;
}

:::danger
Runtime Error

Line 7: Char 25:
runtime error:
left shift of negative value -2147483648 (solution.c)
:::
:::danger
Last executed input
-1
1
:::
-2147483648
is 10000000000000000000000000000000

LeetCode is saying a left-shift to 10000000000000000000000000000000 is incorrect.

## Question 0
Why do I have to add `& 0xFFFFFFFF`?
In fact, the version without `& 0xFFFFFFFF`

int getSum(int a, int b){
    int carry; // 64-bit
    while (b != 0) {
        carry = a & b;
        a = a ^ b;
        b = (carry << 1);
    }
    return a;
}

will work in https://www.onlinegdb.com/online_c_compiler
and https://www.jdoodle.com/c-online-compiler

I didn't find deference between using `& 0xFFFFFFFF` or not when I printed everything linewise on the above two website, by the code below.

#include <stdio.h>

void print2(int x) {
    
    for (int c = 31; c >= 0; c--) {
        
        int k = x >> c;
        
        if (k & 1)
            printf("1");
        else
            printf("0");
    }
    printf("\n");
}

int getSum(int a, int b){
    
    int direct_sum_without_carry = a ^ b; // XOR
    printf("before the while; XOR;  direct_sum_without_carry  %d\n", direct_sum_without_carry);
    print2(direct_sum_without_carry);
    
    int carry = a & b; // AND
    printf("before the while; AND;                     carry  %d\n", carry);
    print2(carry);
    
    while (carry != 0){
        a = direct_sum_without_carry;
        
        //b = carry << 1;
        b = (carry  & 0xFFFFFFFF) << 1;
        
        direct_sum_without_carry = a ^ b; // XOR
        printf("    direct_sum_without_carry  %12d  ", direct_sum_without_carry);
        print2(direct_sum_without_carry);
        
        carry = a & b; // AND
        printf("    carry                     %12d  ", carry);
        print2(carry);
    }
    int direct_sum_with_all_carry = direct_sum_without_carry;
    return direct_sum_with_all_carry;
}
int main() {   
    int a = -1;
    int b = 1;
    printf("%d\n", getSum(a,b));
    return 0;
}

## Question 1
Why there are 32 bits (8 bytes)?
I have used `sizeof(int)` and got `4` on Leetcode and those online compiler.