write your own Atoi function

 implement the function atoi. The function takes a string(str) as argument and converts it to an integer and returns it. 

Example 1:

Input:

str = 123

Output: 123

Example 2:

Input:

str = 21a

Output: -1

Explanation: Output is -1 as all

characters are not digit only.

c++  implementation with explanation:

    int atoi(string str)
{
    //Your code here
    int result=0;   //Stores and returns the integer converted value for str
    int i=0;    //just a current character pointer for string
    int sign = 1;   //multiplied at the end to result to determine if the string is +ve or -ve
    if(str[i]=='-') //Check the sign of string (+ or -)
    {
        sign = -1;
        i++;
    }
    //Now traverse the entire string and convert it into integer
    while(str[i])
    {
        //If any character is not an integer then just return -1
        if(!(str[i]-'0'>=0 && str[i]-'0'<=9))
            return -1;
        //else just keep updating the result
        result = result*10 + str[i]-'0';
        i++;
    }
    return result*sign;

}

Time Complexity: O(n)  ,where n is the length of the string