String to Integer (atoi)

#Deterministic Finite Automata (DFA)
'''
4 states of dfa:
Q0: Initial State(space state)
Q1: sign state
Q2: digit state (digit formation takes place here)
Q3: final halting state
Transitions:
Δ(q0, '\s') = q0         Δ(q1, '\d') = q2.    Δ(q2, '\d') = q2
Δ(q0, '[+-]') = q1       Δ(q1, '\D') = q3.    Δ(q1, '\D') = q3
Δ(q0, '\d') = q2
Δ(q0, '[a-Z][A-Z]') = q3

Reduction: states q1 and q2 can be merged (same transitions)
'''
class Solution:
    res, sgn = 0, 1
    def myAtoi(self, s: str) -> int:
        #transition functions
        def d1(ip):
            if ip.isspace():
                return 'q0'
            elif ip == '-':
                self.sgn = -1
                return 'q1'
            elif ip == '+':
                return 'q1'
            elif ip.isdigit():
                self.res = int(ip)
                return 'q1'
            else:
                return 'q2'
        def d2(ip):
            if ip.isdigit():
                self.res = (self.res<<3) + (self.res<<1) + int(ip)
                return 'q1'
            else:
                return 'q2'

        delta = {'q0': d1, 'q1': d2}
        curr = 'q0' #initial state
        for i in s:
            curr = delta[curr](i)
            if curr == 'q2': #final state
                break
        res = self.sgn*self.res
        res = max(res, -2**31)
        res = min(res, 2**31-1)
        return res

#intuitive way
class Solution:
    def myAtoi(self, s: str) -> int:
        sgn = 1
        i = 0
        n = len(s)
        while i<n and s[i].isspace():#can use lstrip()
            i += 1
        if i == n:
            return 0
        if s[i] == '+':
            i += 1
            sgn = 1
        elif s[i] == '-':
            i += 1
            sgn = -1
        lim1, lim2 = -pow(2, 31), pow(2, 31)-1
        res = 0
        while i<n and s[i].isdigit() and lim1<=res<=lim2:
            res = (res<<3) + (res<<1) +int(s[i])
            i += 1
        res = sgn*res
        if res < lim1:
            return lim1
        elif res > lim2:
            return lim2
        return res