String to Integer (AtoI – ASCII to Integer) – Recursive Solution

Objective: Write a recursive function to implement atoi, which converts string to an integer.

Rules for converting string to an integer.

  • If the string is null or empty, Integer will be 0.
  • The first character of the string may represent the sign of the integer. ‘+’ or ‘-‘
  • Discard the white spaces if necessary.
  • If String contains characters either before digits or after digits, ignore them.


Input String: 1234
Converted Integer: 1234
Input String: -03056
Converted Integer: -3056
Input String: -91283472332
Converted Integer: -2147483648
Input String: 2.56
Converted Integer: 2
Input String: 2147483643
Converted Integer: 2147483643
Input String: 216 after words
Converted Integer: 216
Input String: Before words 325
Converted Integer: 325


  1. check if the string is null, empty or “” then return 0.
  2. Trim the string from both ends.
  3. initialize sign = 1, result=0.
  4. Check if the first character of the input string is either ‘+’ or ‘-‘, start the string process from index 1 and In the case of ‘-‘, do sign = -1 and remove the first character from the string.
  5. Now make a call to the recursive function, this function will calculate the ASCII value for the last character in the string and make to the recursive call with remaining string. Multiply the result of the remaining string with 10 and add ASCII value (calculated earlier) and return. (When adding ASCII, make sure that addition is not crossing MAX_VALUE of Integer, if it does then just return MAX_VALUE)
  6. Multiply the returned result with the sign. This will be the final answer.



Input String: '-1234'
Converted Integer: -1234
Input String: '+555'
Converted Integer: 555
Input String: ' -03056'
Converted Integer: -3056
Input String: '-91283472332'
Converted Integer: -2147483647
Input String: '2.56'
Converted Integer: 256
Input String: '2147483648'
Converted Integer: 2147483647
Input String: '216 after words'
Converted Integer: 216
Input String: 'Before words 325'
Converted Integer: 325

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If you find anything incorrect or you feel that there is any better approach to solve the above problem, please write comment.

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