LC942. DI String Match

class Solution:
    def diStringMatch(self, S: str) -> List[int]:
        """tc O(N) sc O(N)
        1. initialize left ptr and right ptr 
        2. go through S, check if s == 'D' or 'I'
        3. if s == 'I', append left value; otherwise append right value 
        4. add the rest left ptr to the end 
        """
        res = []
        n = len(S)
        l = i = 0
        r = n
        while i <n :
            if S[i] == 'I':
                res.append(l)
                l += 1                 
            elif S[i] == 'D':
                res.append(r)
                r -= 1 
            i += 1    
        res.append(l)
        return res