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Solution 7: Lexical Integer Extraction (atoi)

Approach Explanation

We process the string step-by-step: trimming, checking sign, and then accumulating digits while checking for overflow.

Step-by-Step Logic

  1. s = s.lstrip(). If empty, return 0.
  2. Check first character for + or -. Set sign.
  3. Iterate through characters:
    • If character is a digit, add to res.
    • If not a digit, stop.
  4. Apply sign.
  5. Clamp result to [-2^31, 2^31 - 1].

Complexity

  • Time Complexity: O(N).
  • Space Complexity: O(1).

Code

def my_atoi(s):
    s = s.strip()
    if not s: return 0
    
    sign = 1
    i = 0
    if s[0] == '-':
        sign = -1
        i = 1
    elif s[0] == '+':
        i = 1
        
    res = 0
    while i < len(s) and s[i].isdigit():
        res = res * 10 + int(s[i])
        i += 1
        
    res *= sign
    
    # Clamping
    INT_MAX = 2**31 - 1
    INT_MIN = -2**31
    if res > INT_MAX: return INT_MAX
    if res < INT_MIN: return INT_MIN
    
    return res