https://leetcode.com/problems/string-to-integer-atoi/

string-to-integer-atoi.java

class Solution {
    public int myAtoi(String str) {
        if (str == null || str.length() == 0) {
            return 0;
        }
        
        int ptr = 0;
        int len = str.length();
        while(ptr < len && str.charAt(ptr) == ' ') {
            ptr++;
        }
        
        int sign = 1;
        
        if (ptr < len && !Character.isDigit(str.charAt(ptr))) {
            if (str.charAt(ptr) == '+') {
                sign = 1;
            } else if (str.charAt(ptr) == '-') {
                sign = -1;
            } else {
                return 0;
            }
            ptr++;
        }
        
        int res = 0;
        while(ptr < len) {
            char c = str.charAt(ptr);
            if (c < '0' || c > '9') {
                break;
            }
            int num = c - '0';
            if (Integer.MAX_VALUE / 10 < res || (Integer.MAX_VALUE - res * 10 ) < num) {
                return sign == 1 ? Integer.MAX_VALUE : Integer.MIN_VALUE;
            }
            res = res * 10 + num;
            ptr++;
        }
        
        return sign * res;
    }
}