Today I have done leetcode's September LeetCoding Challenge with cpp.

September LeetCoding Challenge 27

Description

Decoded String at Index

You are given an encoded string s. To decode the string to a tape, the encoded string is read one character at a time and the following steps are taken:

• If the character read is a letter, that letter is written onto the tape.
• If the character read is a digit d, the entire current tape is repeatedly written d - 1 more times in total.

Given an integer k, return the kth letter (1-indexed) in the decoded string.

Example 1:

Input: s = "leet2code3", k = 10
Output: "o"
Explanation: The decoded string is "leetleetcodeleetleetcodeleetleetcode".
The 10th letter in the string is "o".

Example 2:

Input: s = "ha22", k = 5
Output: "h"
Explanation: The decoded string is "hahahaha".
The 5th letter is "h".

Example 3:

Input: s = "a2345678999999999999999", k = 1
Output: "a"
Explanation: The decoded string is "a" repeated 8301530446056247680 times.
The 1st letter is "a".

Constraints:

• 2 <= s.length <= 100
• s consists of lowercase English letters and digits 2 through 9.
• s starts with a letter.
• 1 <= k <= 109
• It is guaranteed that k is less than or equal to the length of the decoded string.
• The decoded string is guaranteed to have less than 263 letters.

Solution

class Solution {
public:
  string decodeAtIndex(string S, int K) {
    char prevChar = S[0];
    int len = S.length();
    vector<unsigned long long> cnt(len);
    vector<char> lastChar(len);
    cnt[0] = 1, lastChar[0] = S[0];
    for(int i = 1; i < len; ++i) {
      if(isdigit(S[i])) {
        cnt[i] = cnt[i-1] * (S[i]-'0');
      } else {
        cnt[i] = cnt[i-1] + 1;
        prevChar = S[i];
      }
      lastChar[i] = prevChar;
    }
    if(K >= cnt.back()) K %= cnt.back();
    if(K == 0) return string(1, prevChar);
    while(K) {
      int pos = 0;
      while(pos < len && cnt[pos] < K) ++pos;
      if(cnt[pos] == K || K % cnt[pos-1] == 0) return string(1, lastChar[pos]);
      K %= cnt[pos-1];
    }
    return "";
  }
};

// Accepted
// 45/45 cases passed (0 ms)
// Your runtime beats 100 % of cpp submissions
// Your memory usage beats 7.16 % of cpp submissions (6.6 MB)