Lexicographically Smallest String After Substring Operation
<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; String
class Solution:
    def smallestString(self, s: str) -> str:
        n = len(s)
        i = 0
        while i < n and s[i] == "a":
            i += 1
        if i == n:
            return s[:-1] + "z"
        j = i
        while j < n and s[j] != "a":
            j += 1
        return s[:i] + "".join(chr(ord(c) - 1) for c in s[i:j]) + s[j:]
