8. String to Integer (atoi).go

// Implement the myAtoi(string s) function, which converts a string to a 32-bit signed integer.

// The algorithm for myAtoi(string s) is as follows:

//     Whitespace: Ignore any leading whitespace (" ").
//     Signedness: Determine the sign by checking if the next character is '-' or '+', assuming positivity is neither present.
//     Conversion: Read the integer by skipping leading zeros until a non-digit character is encountered or the end of the string is reached. If no digits were read, then the result is 0.
//     Rounding: If the integer is out of the 32-bit signed integer range [-231, 231 - 1], then round the integer to remain in the range. Specifically, integers less than -231 should be rounded to -231, and integers greater than 231 - 1 should be rounded to 231 - 1.

// Return the integer as the final result.

 

// Example 1:

// Input: s = "42"

// Output: 42

// Explanation:

// The underlined characters are what is read in and the caret is the current reader position.
// Step 1: "42" (no characters read because there is no leading whitespace)
//          ^
// Step 2: "42" (no characters read because there is neither a '-' nor '+')
//          ^
// Step 3: "42" ("42" is read in)
//            ^

// Example 2:

// Input: s = " -042"

// Output: -42

// Explanation:

// Step 1: "   -042" (leading whitespace is read and ignored)
//             ^
// Step 2: "   -042" ('-' is read, so the result should be negative)
//              ^
// Step 3: "   -042" ("042" is read in, leading zeros ignored in the result)
//                ^

// Example 3:

// Input: s = "1337c0d3"

// Output: 1337

// Explanation:

// Step 1: "1337c0d3" (no characters read because there is no leading whitespace)
//          ^
// Step 2: "1337c0d3" (no characters read because there is neither a '-' nor '+')
//          ^
// Step 3: "1337c0d3" ("1337" is read in; reading stops because the next character is a non-digit)
//              ^

// Example 4:

// Input: s = "0-1"

// Output: 0

// Explanation:

// Step 1: "0-1" (no characters read because there is no leading whitespace)
//          ^
// Step 2: "0-1" (no characters read because there is neither a '-' nor '+')
//          ^
// Step 3: "0-1" ("0" is read in; reading stops because the next character is a non-digit)
//           ^

// Example 5:

// Input: s = "words and 987"

// Output: 0

// Explanation:

// Reading stops at the first non-digit character 'w'.

 

// Constraints:

//     0 <= s.length <= 200
//     s consists of English letters (lower-case and upper-case), digits (0-9), ' ', '+', '-', and '.'.



package main

import("fmt")

func strToInt(s string)int{

	keyMapping:=map[rune]int{'0':0,'1':1,'2':2,'3':3,'4':4,'5':5,'6':6,'7':7,'8':8,'9':9}

	var res int;
	neg:=false
	firstCharFound:=false
	for _,v:=range s{
		if v=='-' && !firstCharFound {
            neg=true;firstCharFound=true;continue
		}else if v==' '{
            if firstCharFound{break};
            continue
            }else if v=='+' && !firstCharFound{
                firstCharFound=true
                continue
            }else if _,ok:=keyMapping[v];!ok{break}


		if _,ok:=keyMapping[v];ok &&!firstCharFound{firstCharFound=true}

		if firstCharFound{
			if _,ok:=keyMapping[v];!ok{break}
			res=(res*10)+keyMapping[v]
            if res >= 2147483648 && !neg{
               return 2147483647
            }else if res >= 2147483648 && neg{
				return -2147483648
			}
		}
	}

	if neg{res=-res} //changing to negetive value


	return res
}


func main(){

	inp:="-9223372036854775809"

	fmt.Println(strToInt(inp))

}