String to Integer (atoi)

1. str trim 并且trim 后检查是不是length == 0

2. check 不是数字的时候 返回当前的结果乘以sign

Description

Implement function atoi to convert a string to an integer.

If no valid conversion could be performed, a zero value is returned.

If the correct value is out of the range of representable values, INT_MAX (2147483647) or INT_MIN (-2147483648) is returned.Have you met this question in a real interview? Yes

Example

"10" => 10
"-1" => -1
"123123123123123" => 2147483647
"1.0" => 1

public class Solution {
    /**
     * @param str: A string
     * @return: An integer
     */
    public int atoi(String str) {
        // write your code here
        
        
        if(str == null ) return 0;
        
        str = str.trim();
        
        if(str.length() == 0) return 0;
        
        int sign = 1, index = 0;
        
        if(str.charAt(index) == '+'){
            index++;
        }else if(str.charAt(index) == '-'){
            sign = -1;
            index++;
        }
        long num = 0;
        for (;index < str.length() ;index++ ){
            //not num
            
            if(str.charAt(index) < '0' || str.charAt(index) > '9'){
                break;
            }
            
            num = num * 10 + (str.charAt(index) -'0');
            if(num > Integer.MAX_VALUE){
                break;
            }
            
        }
        
        if(num * sign < Integer.MIN_VALUE){
            return Integer.MIN_VALUE;
        }
        
        if(num * sign > Integer.MAX_VALUE){
            return Integer.MAX_VALUE;
        }
        
        return (int)num*sign;
    }
}

二刷 时间复杂度 o(n), space o(1)

class Solution {
    public int myAtoi(String str) {
        //corner case " + -" not valid . out bounday
        if(str == null || str.length() == 0)
            return 0;
        
        str = str.trim();
        if(str.length() == 0){
            return 0;
        }
        int sign = 1, start = 0;
        long res = 0;
        
        char f = str.charAt(0);
        
        if(f == '+'){
            sign =1;
            start++;
        }
        
        if(f == '-'){
            sign = -1;
            start++;
        }
        
        for(int i = start; i< str.length() ; i++){
            if(!Character.isDigit(str.charAt(i))){
                return (int) res*sign;
            }
            
            res = res*10 + str.charAt(i) - '0';
            
            if(res > Integer.MAX_VALUE){
                return sign == 1 ? Integer.MAX_VALUE : Integer.MIN_VALUE;
            }
        }
        
        return (int) res*sign;
    }
}