2734. Lexicographically Smallest String After Substring Operation
Description
Given a string s
consisting of lowercase English letters. Perform the following operation:
- Select any non-empty substring then replace every letter of the substring with the preceding letter of the English alphabet. For example, 'b' is converted to 'a', and 'a' is converted to 'z'.
Return the lexicographically smallest string after performing the operation.
Example 1:
Input: s = "cbabc"
Output: "baabc"
Explanation:
Perform the operation on the substring starting at index 0, and ending at index 1 inclusive.
Example 2:
Input: s = "aa"
Output: "az"
Explanation:
Perform the operation on the last letter.
Example 3:
Input: s = "acbbc"
Output: "abaab"
Explanation:
Perform the operation on the substring starting at index 1, and ending at index 4 inclusive.
Example 4:
Input: s = "leetcode"
Output: "kddsbncd"
Explanation:
Perform the operation on the entire string.
Constraints:
1 <= s.length <= 3 * 105
s
consists of lowercase English letters
Solutions
Solution 1: Greedy Algorithm
We can traverse the string $s$ from left to right, find the position $i$ of the first character that is not 'a', and then find the position $j$ of the first 'a' character starting from $i$. We decrement each character in $s[i:j]$, and finally return the processed string.
The time complexity is $O(n)$, and the space complexity is $O(n)$. Here, $n$ is the length of the string $s$.
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