50 lines
1 KiB
Java
50 lines
1 KiB
Java
class Solution {
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private static final int EMPTY = -1;
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private static int charToDigit(String s, int i) {
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return s.charAt(i) - '0';
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}
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private int numDecodings(String s, int[] dp, int i) {
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// out of bounds
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if (i < 0 || i > s.length()) {
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return 0;
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}
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// final digit
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if (i == s.length()) {
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return 1;
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}
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// starting with 0, not allowed
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if (s.charAt(i) == '0') {
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return 0;
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}
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// solution has been already precomputed
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if (dp[i] != EMPTY) {
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return dp[i];
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}
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// taking just one digit
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int result = numDecodings(s, dp, i + 1);
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// taking two digits
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if (i + 1 < s.length() && charToDigit(s, i) * 10 + charToDigit(s, i + 1) <= 26) {
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result += numDecodings(s, dp, i + 2);
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}
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// save and return
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dp[i] = result;
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return result;
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}
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public int numDecodings(String s) {
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int[] dp = new int[s.length()];
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for (int i = 0; i < dp.length; ++i) {
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dp[i] = EMPTY;
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}
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return numDecodings(s, dp, 0);
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}
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}
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