880. Decoded String at Index

Medium (Trung bình) C++ 🔗 Xem trên LeetCode

📋 Đề Bài

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.

🧠 Thuật Toán & Kỹ Thuật

String (Chuỗi)
⏱️ Thời gian O(n²)
💾 Không gian O(1)

💻 Lời Giải

C++ 0880-decoded-string-at-index.cpp 
class Solution {
public:
    string decodeAtIndex(string s, int k) {
        const int n = s.size();
        long long real_size = 0;
        
        for (char c : s) {
            if (isdigit(c)) {
                real_size = real_size * (c - '0');
            }
            else {
                real_size++;
            }
        }
        
        for (int i = n - 1; i >= 0; --i) {
            char c = s[i];
            
            if (isdigit(c)) {
                real_size /= (c - '0');
                k %= real_size;
            }
            else {
                if (k % real_size == 0) {
                    return string(1, c);
                }
                real_size--;
            }
        }
        
        return "";
    }
};