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package class30;
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public class Problem_0091_DecodeWays {
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public static int numDecodings1(String s) {
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if (s == null || s.length() == 0) {
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return 0;
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}
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char[] str = s.toCharArray();
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return process(str, 0);
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}
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// 潜台词：str[0...index-1]已经转化完了，不用操心了
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// str[index....] 能转出多少有效的，返回方法数
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public static int process(char[] str, int index) {
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if (index == str.length) {
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return 1;
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}
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if (str[index] == '0') {
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return 0;
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}
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int ways = process(str, index + 1);
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if (index + 1 == str.length) {
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return ways;
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}
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int num = (str[index] - '0') * 10 + str[index + 1] - '0';
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if (num < 27) {
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ways += process(str, index + 2);
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}
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return ways;
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}
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public static int numDecodings2(String s) {
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if (s == null || s.length() == 0) {
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return 0;
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}
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char[] str = s.toCharArray();
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int N = str.length;
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// dp[i] -> process(str, index)返回值 index 0 ~ N
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int[] dp = new int[N + 1];
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dp[N] = 1;
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// dp依次填好 dp[i] dp[i+1] dp[i+2]
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for (int i = N - 1; i >= 0; i--) {
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if (str[i] != '0') {
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dp[i] = dp[i + 1];
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if (i + 1 == str.length) {
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continue;
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}
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int num = (str[i] - '0') * 10 + str[i + 1] - '0';
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if (num <= 26) {
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dp[i] += dp[i + 2];
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}
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}
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}
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return dp[0];
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}
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public static int numDecodings(String s) {
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if (s == null || s.length() == 0) {
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return 0;
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}
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char[] str = s.toCharArray();
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int N = str.length;
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int[] dp = new int[N + 1];
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dp[N] = 1;
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for (int i = N - 1; i >= 0; i--) {
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if (str[i] == '0') {
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dp[i] = 0;
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} else if (str[i] == '1') {
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dp[i] = dp[i + 1];
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if (i + 1 < N) {
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dp[i] += dp[i + 2];
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}
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} else if (str[i] == '2') {
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dp[i] = dp[i + 1];
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if (i + 1 < str.length && (str[i + 1] >= '0' && str[i + 1] <= '6')) {
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dp[i] += dp[i + 2];
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}
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} else {
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dp[i] = dp[i + 1];
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}
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}
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return dp[0];
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}
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}
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