Your task is to implement the function atoi. The function takes a string(str) as argument and converts it to an integer and returns it.
Note: You are not allowed to use inbuilt function.
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.
Your Task:
Complete the function atoi() which takes a string as input parameter and returns integer value of it. if the input string is not a numerical string then returns -1.
Note: Numerical strings are the string where either every character is in between 0-9 or where the first character is '-' and the rest all characters are in between 0-9.
Expected Time Complexity: O(|S|), |S| = length of string str.
Expected Auxiliary Space: O(1)
Constraints:
1 ≤ length of S ≤ 10
Note:The Input/Ouput format and Example given are used for system's internal purpose, and should be used by a user for Expected Output only. As it is a function problem, hence a user should not read any input from stdin/console. The task is to complete the function specified, and not to write the full code.

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