Java: String to Integer (atoi)

Thought Process

To convert a string to an integer, we need to handle leading whitespaces, sign characters ('+' or '-'), and numeric characters. We iterate through the string, skipping leading spaces, and then check for a sign. After that, we process the numeric characters to build the integer. If the result exceeds the 32-bit signed integer range, we clamp it to INT_MIN or INT_MAX. This ensures we handle edge cases and invalid inputs gracefully. 

class Solution {
    public int myAtoi(String s) {

        boolean isFirstCharacter = true;
        int sign = 1;
        long num =0;
        for(int i=0;i<s.length();i++){

            char ch = s.charAt(i);
            if(ch==' '){ //"334234 3423";

                if(!isFirstCharacter)
                    break;
            }
            else if(ch=='+'){

                if(!isFirstCharacter)
                    break;
                else{
                    sign = 1;
                    isFirstCharacter = false;
                }
            }
            else if(ch=='-'){
                if(!isFirstCharacter)
                    break;
                else{
                    sign = -1;
                    isFirstCharacter = false;
                }
            }
            else if(Character.isDigit(ch)){
                num = num*10 + ch-'0';
                if(num*sign>Integer.MAX_VALUE)
                    return Integer.MAX_VALUE;
                else if(num*sign < Integer.MIN_VALUE)
                    return Integer.MIN_VALUE;

                isFirstCharacter = false;
            }
            else
                break;
        }
        return (int)num*sign;
    }
}

Code Complexity

Time Complexity: O(n)

The algorithm iterates through the string once, making it linear in time complexity.

Space Complexity: O(1)

The algorithm uses a constant amount of extra space, regardless of the input size.

Code copied!