<p>Given a string <code>s</code> consisting of lowercase English letters. Perform the following operation:</p>

<ul>
	<li>Select any non-empty <span data-keyword="substring-nonempty">substring</span> then replace every letter of the substring with the preceding letter of the English alphabet. For example, &#39;b&#39; is converted to &#39;a&#39;, and &#39;a&#39; is converted to &#39;z&#39;.</li>
</ul>

<p>Return the <span data-keyword="lexicographically-smaller-string"><strong>lexicographically smallest</strong></span> string <strong>after performing the operation</strong>.</p>

<p>&nbsp;</p>
<p><strong class="example">Example 1:</strong></p>

<div class="example-block">
<p><strong>Input:</strong> <span class="example-io">s = &quot;cbabc&quot;</span></p>

<p><strong>Output:</strong> <span class="example-io">&quot;baabc&quot;</span></p>

<p><strong>Explanation:</strong></p>

<p>Perform the operation on the substring starting at index 0, and ending at index 1 inclusive.</p>
</div>

<p><strong class="example">Example 2:</strong></p>

<div class="example-block">
<p><strong>Input:</strong> <span class="example-io">s = &quot;aa&quot;</span></p>

<p><strong>Output:</strong> <span class="example-io">&quot;az&quot;</span></p>

<p><strong>Explanation:</strong></p>

<p>Perform the operation on the last letter.</p>
</div>

<p><strong class="example">Example 3:</strong></p>

<div class="example-block">
<p><strong>Input:</strong> <span class="example-io">s = &quot;acbbc&quot;</span></p>

<p><strong>Output:</strong> <span class="example-io">&quot;abaab&quot;</span></p>

<p><strong>Explanation:</strong></p>

<p>Perform the operation on the substring starting at index 1, and ending at index 4 inclusive.</p>
</div>

<p><strong class="example">Example 4:</strong></p>

<div class="example-block">
<p><strong>Input:</strong> <span class="example-io">s = &quot;leetcode&quot;</span></p>

<p><strong>Output:</strong> <span class="example-io">&quot;kddsbncd&quot;</span></p>

<p><strong>Explanation:</strong></p>

<p>Perform the operation on the entire string.</p>
</div>

<p>&nbsp;</p>
<p><strong>Constraints:</strong></p>

<ul>
	<li><code>1 &lt;= s.length &lt;= 3 * 10<sup>5</sup></code></li>
	<li><code>s</code> consists of lowercase English letters</li>
</ul>

'''
Greedy

TC: O(n) SC: O(n)

- substring: A substring is a contiguous non-empty sequence of characters within a string.

From this observation, we want to find the first substring that doesn't contain an 'a' and replace this substring
w/ all prev chars. Why the first substring? Well, that's because those chars in the beginning will lead to a lower overall
value compared to changing those in the back or 'larger len' substrings.
- One edge case: If all chars in substring only consist of 'a'.
'''
class Solution:
    def smallestString(self, s: str) -> str:
        # We split the str by 'a' as to get every substring that doesn't have an 'a'.
        substrings = s.split('a')

        for i, substr in enumerate(substrings):
            # Check if non empty (greedy)
            if len(substr) > 0:
                # Replace each char in this substring w/ preceding char in alphabet
                precedingSubstr = ''.join([chr(ord(char) - 1) for char in substr])
                substrings[i] = precedingSubstr
                break
        # Couldn't replace, meaning, string only consists of 'a'
        else:
            return s[:-1] + 'z'
        return 'a'.join(substrings)
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