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71 lines
4.1 KiB
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71 lines
4.1 KiB
HTML
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<p>You are given a string <code>s</code> and an integer <code>k</code>.</p>
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<p>A <strong>k-subsequence</strong> is a <strong>subsequence</strong> of <code>s</code>, having length <code>k</code>, and all its characters are <strong>unique</strong>, <strong>i.e</strong>., every character occurs once.</p>
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<p>Let <code>f(c)</code> denote the number of times the character <code>c</code> occurs in <code>s</code>.</p>
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<p>The <strong>beauty</strong> of a <strong>k-subsequence</strong> is the <strong>sum</strong> of <code>f(c)</code> for every character <code>c</code> in the k-subsequence.</p>
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<p>For example, consider <code>s = "abbbdd"</code> and <code>k = 2</code>:</p>
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<ul>
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<li><code>f('a') = 1</code>, <code>f('b') = 3</code>, <code>f('d') = 2</code></li>
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<li>Some k-subsequences of <code>s</code> are:
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<ul>
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<li><code>"<u><strong>ab</strong></u>bbdd"</code> -> <code>"ab"</code> having a beauty of <code>f('a') + f('b') = 4</code></li>
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<li><code>"<u><strong>a</strong></u>bbb<strong><u>d</u></strong>d"</code> -> <code>"ad"</code> having a beauty of <code>f('a') + f('d') = 3</code></li>
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<li><code>"a<strong><u>b</u></strong>bb<u><strong>d</strong></u>d"</code> -> <code>"bd"</code> having a beauty of <code>f('b') + f('d') = 5</code></li>
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</ul>
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</li>
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</ul>
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<p>Return <em>an integer denoting the number of k-subsequences </em><em>whose <strong>beauty</strong> is the <strong>maximum</strong> among all <strong>k-subsequences</strong></em>. Since the answer may be too large, return it modulo <code>10<sup>9</sup> + 7</code>.</p>
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<p>A subsequence of a string is a new string formed from the original string by deleting some (possibly none) of the characters without disturbing the relative positions of the remaining characters.</p>
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<p><strong>Notes</strong></p>
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<ul>
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<li><code>f(c)</code> is the number of times a character <code>c</code> occurs in <code>s</code>, not a k-subsequence.</li>
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<li>Two k-subsequences are considered different if one is formed by an index that is not present in the other. So, two k-subsequences may form the same string.</li>
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</ul>
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<p> </p>
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<p><strong class="example">Example 1:</strong></p>
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<pre>
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<strong>Input:</strong> s = "bcca", k = 2
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<strong>Output:</strong> 4
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<strong>Explanation:</strong> <span style="white-space: normal">From s we have f('a') = 1, f('b') = 1, and f('c') = 2.</span>
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The k-subsequences of s are:
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<strong><u>bc</u></strong>ca having a beauty of f('b') + f('c') = 3
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<strong><u>b</u></strong>c<u><strong>c</strong></u>a having a beauty of f('b') + f('c') = 3
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<strong><u>b</u></strong>cc<strong><u>a</u></strong> having a beauty of f('b') + f('a') = 2
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b<strong><u>c</u></strong>c<u><strong>a</strong></u><strong> </strong>having a beauty of f('c') + f('a') = 3
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bc<strong><u>ca</u></strong> having a beauty of f('c') + f('a') = 3
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There are 4 k-subsequences that have the maximum beauty, 3.
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Hence, the answer is 4.
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</pre>
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<p><strong class="example">Example 2:</strong></p>
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<pre>
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<strong>Input:</strong> s = "abbcd", k = 4
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<strong>Output:</strong> 2
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<strong>Explanation:</strong> From s we have f('a') = 1, f('b') = 2, f('c') = 1, and f('d') = 1.
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The k-subsequences of s are:
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<u><strong>ab</strong></u>b<strong><u>cd</u></strong> having a beauty of f('a') + f('b') + f('c') + f('d') = 5
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<u style="white-space: normal;"><strong>a</strong></u>b<u><strong>bcd</strong></u> having a beauty of f('a') + f('b') + f('c') + f('d') = 5
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There are 2 k-subsequences that have the maximum beauty, 5.
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Hence, the answer is 2.
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</pre>
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<p> </p>
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<p><strong>Constraints:</strong></p>
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<ul>
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<li><code>1 <= s.length <= 2 * 10<sup>5</sup></code></li>
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<li><code>1 <= k <= s.length</code></li>
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<li><code>s</code> consists only of lowercase English letters.</li>
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</ul>
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