Leetcode-8.java

class Solution {
    public int myAtoi(String s) {
        if(s == null || s.length() == 0) return 0;

        final int INT_MAX = Integer.MAX_VALUE;
        final int INT_MIN = Integer.MIN_VALUE;

        int i = 0;
        int n = s.length();

        while( i < n && s.charAt(i) == ' ') i++;

        if( i == n) return 0;

        int sign = 1;
        if (s.charAt(i) == '+'){
            i++;
        }else if (s.charAt(i) == '-'){
            sign = -1;
            i++;
        }

        long res = 0;
        while ( i < n && Character.isDigit(s.charAt(i))) {
            int digit = s.charAt(i) - '0';
            res = res * 10 + digit; //key

            if ( sign * res <= INT_MIN) return INT_MIN;
            if ( sign * res >= INT_MAX) return INT_MAX;

            i++;
        }

        return (int)(res * sign);
    }
}