Problem
Implement atoi which converts a string to an integer.
The function first discards as many whitespace characters as necessary until the first non-whitespace character is found. Then, starting from this character, takes an optional initial plus or minus sign followed by as many numerical digits as possible, and interprets them as a numerical value.
The string can contain additional characters after those that form the integral number, which are ignored and have no effect on the behavior of this function.
If the first sequence of non-whitespace characters in str is not a valid integral number, or if no such sequence exists because either str is empty or it contains only whitespace characters, no conversion is performed.
If no valid conversion could be performed, a zero value is returned.
Note:
Only the space character ' ' is considered as whitespace character.
Assume we are dealing with an environment which could only store integers within the 32-bit signed integer range: [−231, 231 − 1]. If the numerical value is out of the range of representable values, INT_MAX (2^31 − 1) or INT_MIN (−2^31) is returned.
Example 1:
Input: "42"
Output: 42
Example 2:
Input: " -42"
Output: -42
Explanation: The first non-whitespace character is '-', which is the minus sign.
Then take as many numerical digits as possible, which gets 42.
Example 3:
Input: "4193 with words"
Output: 4193
Explanation: Conversion stops at digit '3' as the next character is not a numerical digit.
Example 4:
Input: "words and 987"
Output: 0
Explanation: The first non-whitespace character is 'w', which is not a numerical
digit or a +/- sign. Therefore no valid conversion could be performed.
Example 5:
Input: "-91283472332"
Output: -2147483648
Explanation: The number "-91283472332" is out of the range of a 32-bit signed integer.
Thefore INT_MIN (−2^31) is returned.
Solution
This problem is kind of straight forward, but with lots of edge conditions. We have 3 states. state 0, remove white space; state 1, read in numbers.
At state 0, if - or digits is met, the state is changed to state 1; if a none whitespace or digits are met, we return 0.
At state 1, if none digit character is met, we stop read further, and return the parsed number.
public class Atoi {
public static int convert(String str) {
// -42 with word
String number = "";//-42
char[] strc = str.toCharArray();
int flag = 0;
for(int i = 0; i < str.length(); i++) {
char c = strc[i];
if(flag == 0) {
if(c == '-' || (c - '0' < 9 && c -'0' >=0)) {
flag = 1;
number += c;
continue;
} else if(c == ' ') {
//do nothing
} else {
return 0;
}
} else if(flag == 1) {
if(c - '0' < 9 && c -'0' >=0) {
number += c;
} else {
return parse(number);
}
}
}
return 0;
}
private static int parse(String number) {//-42
if(inRange(number))
return Integer.parseInt(number);
else if(number.charAt(0) == '-')
return Integer.MIN_VALUE;
else
return Integer.MAX_VALUE;
}
private static boolean inRange(String number) {
if(number.charAt(0) == '-') {
String minInt = "2147483648";
if(number.substring(1).length() > minInt.length()) return false;
else if(number.substring(1).length() < minInt.length()) return true;
else {
for(int i = 0; i < minInt.length(); i++) {
if(number.charAt(i) - minInt.charAt(i) < 0)
return true;
else if(number.charAt(i) - minInt.charAt(i) > 0)
return false;
}
return true;
}
} else {
String maxInt = "2147483647";
if(number.length() > maxInt.length()) return false;
else if(number.length() < maxInt.length()) return true;
else {
for(int i = 0; i < maxInt.length(); i++) {
if(number.charAt(i) - maxInt.charAt(i) < 0)
return true;
else if(number.charAt(i) - maxInt.charAt(i) > 0)
return false;
}
return true;
}
}
}
public static void main(String...args) {
System.out.println(convert(" -42 with word"));
System.out.println(convert(" -423345324664544343 with word"));
System.out.println(convert("words and 987"));
}
}
public static int convert(String str) {
// -42 with word
String number = "";//-42
char[] strc = str.toCharArray();
int flag = 0;
for(int i = 0; i < str.length(); i++) {
char c = strc[i];
if(flag == 0) {
if(c == '-' || (c - '0' < 9 && c -'0' >=0)) {
flag = 1;
number += c;
continue;
} else if(c == ' ') {
//do nothing
} else {
return 0;
}
} else if(flag == 1) {
if(c - '0' < 9 && c -'0' >=0) {
number += c;
} else {
return parse(number);
}
}
}
return 0;
}
private static int parse(String number) {//-42
if(inRange(number))
return Integer.parseInt(number);
else if(number.charAt(0) == '-')
return Integer.MIN_VALUE;
else
return Integer.MAX_VALUE;
}
private static boolean inRange(String number) {
if(number.charAt(0) == '-') {
String minInt = "2147483648";
if(number.substring(1).length() > minInt.length()) return false;
else if(number.substring(1).length() < minInt.length()) return true;
else {
for(int i = 0; i < minInt.length(); i++) {
if(number.charAt(i) - minInt.charAt(i) < 0)
return true;
else if(number.charAt(i) - minInt.charAt(i) > 0)
return false;
}
return true;
}
} else {
String maxInt = "2147483647";
if(number.length() > maxInt.length()) return false;
else if(number.length() < maxInt.length()) return true;
else {
for(int i = 0; i < maxInt.length(); i++) {
if(number.charAt(i) - maxInt.charAt(i) < 0)
return true;
else if(number.charAt(i) - maxInt.charAt(i) > 0)
return false;
}
return true;
}
}
}
public static void main(String...args) {
System.out.println(convert(" -42 with word"));
System.out.println(convert(" -423345324664544343 with word"));
System.out.println(convert("words and 987"));
}
}
The time cost is O(N) and space cost is O(1).
