CareerCup

read each character and get its numerical equivalent by subracting the ascii value of '0' from the ascii value of the character.
build the number using a loop and multiplying by 10 and adding it to the already formed number.
reverse the number at the end(since we read the number in reverse from the char*)

@bob, instead of reversing the number at the end you can also, calculate the number of digits beforehand, and set values by decrementing a pointer.

also, be sure to take into account negative numbers if the interviewer wants.

nugarp is correct, scan the char array from left to right, check for the first negative sign, then for the rest of characters, sum = sum * 10 + (array[i] - '0'), negate at the end if the first char is '-'

int atoi(char* numberstr){
    int num=0;
    bool sign  =false;
    int length = getLength(numberstr);
    if(numberstr[0]='-'){
        sign = true;
        numberstr++;
        length--;
    }        
    while(*numberstr){
        num = (*numberstr - '0')*(length--);
        numberstr++;
    }
    
    if(sign) num = num*(-1);
    
    return num;
}

The code above reads the number from the string from left to right, setting the sign flag if the first character is '-'.

<pre lang="" line="1" title="CodeMonkey82292" class="run-this">#include <stdio.h>
#include <ctype.h>
#include <errno.h>

int myatoi(const char *src)
{
int c;
int total;
int sign;
int overflow = 0;

int max = 0x7FFFFFFF / 10;

while(isspace((int)(unsigned char)*src))
++src;
c = (int)(unsigned char)*src++;
sign = c;
if(c == '-' || c == '+')
c = (int)(unsigned char)*src++;
total = 0;
while(isdigit(c))
{
if(total > max) { overflow = 1; break; }
total = 10 * total + (c - '0');
c = (int)(unsigned char)*src++;
}

if(overflow) {
errno = ERANGE;
return 0x7FFFFFFF;
}

if(sign == '-')
return -total;
else
return total;
}

int main() {
char src[100];
while(scanf("%s", src))
printf("%d\n", myatoi(src));
}
</pre><pre title="CodeMonkey82292" input="yes">
</pre>

i think this can work too. i have also tested this and this worked. if there is any mistake then please point it out. all constructive comments are welcome.

public static int atoi(String a){
	char[] arr = a.toCharArray();
	int result = 0;
	for(int i=0; i<arr.length; i++){
		int k = (int) arr[i];
		k = k-48;
		result = (result*10)+k;
	}
	return result;
}