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63 lines
3.9 KiB
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63 lines
3.9 KiB
HTML
<p>A message containing letters from <code>A-Z</code> can be <strong>encoded</strong> into numbers using the following mapping:</p>
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<pre>
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'A' -> "1"
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'B' -> "2"
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...
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'Z' -> "26"
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</pre>
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<p>To <strong>decode</strong> an encoded message, all the digits must be grouped then mapped back into letters using the reverse of the mapping above (there may be multiple ways). For example, <code>"11106"</code> can be mapped into:</p>
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<ul>
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<li><code>"AAJF"</code> with the grouping <code>(1 1 10 6)</code></li>
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<li><code>"KJF"</code> with the grouping <code>(11 10 6)</code></li>
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</ul>
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<p>Note that the grouping <code>(1 11 06)</code> is invalid because <code>"06"</code> cannot be mapped into <code>'F'</code> since <code>"6"</code> is different from <code>"06"</code>.</p>
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<p><strong>In addition</strong> to the mapping above, an encoded message may contain the <code>'*'</code> character, which can represent any digit from <code>'1'</code> to <code>'9'</code> (<code>'0'</code> is excluded). For example, the encoded message <code>"1*"</code> may represent any of the encoded messages <code>"11"</code>, <code>"12"</code>, <code>"13"</code>, <code>"14"</code>, <code>"15"</code>, <code>"16"</code>, <code>"17"</code>, <code>"18"</code>, or <code>"19"</code>. Decoding <code>"1*"</code> is equivalent to decoding <strong>any</strong> of the encoded messages it can represent.</p>
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<p>Given a string <code>s</code> consisting of digits and <code>'*'</code> characters, return <em>the <strong>number</strong> of ways to <strong>decode</strong> it</em>.</p>
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<p>Since the answer may be very large, return it <strong>modulo</strong> <code>10<sup>9</sup> + 7</code>.</p>
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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 = "*"
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<strong>Output:</strong> 9
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<strong>Explanation:</strong> The encoded message can represent any of the encoded messages "1", "2", "3", "4", "5", "6", "7", "8", or "9".
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Each of these can be decoded to the strings "A", "B", "C", "D", "E", "F", "G", "H", and "I" respectively.
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Hence, there are a total of 9 ways to decode "*".
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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 = "1*"
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<strong>Output:</strong> 18
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<strong>Explanation:</strong> The encoded message can represent any of the encoded messages "11", "12", "13", "14", "15", "16", "17", "18", or "19".
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Each of these encoded messages have 2 ways to be decoded (e.g. "11" can be decoded to "AA" or "K").
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Hence, there are a total of 9 * 2 = 18 ways to decode "1*".
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</pre>
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<p><strong class="example">Example 3:</strong></p>
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<pre>
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<strong>Input:</strong> s = "2*"
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<strong>Output:</strong> 15
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<strong>Explanation:</strong> The encoded message can represent any of the encoded messages "21", "22", "23", "24", "25", "26", "27", "28", or "29".
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"21", "22", "23", "24", "25", and "26" have 2 ways of being decoded, but "27", "28", and "29" only have 1 way.
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Hence, there are a total of (6 * 2) + (3 * 1) = 12 + 3 = 15 ways to decode "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 <= 10<sup>5</sup></code></li>
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<li><code>s[i]</code> is a digit or <code>'*'</code>.</li>
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</ul>
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