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package class18;
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public class Code05_ConvertToLetterString {
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public static int number(String str) {
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if (str == null || str.length() == 0) {
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return 0;
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}
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return process(str.toCharArray(), 0);
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}
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// str[0...i-1]已经转化完了，固定了
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// i之前的位置，如何转化已经做过决定了, 不用再关心
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// i... 有多少种转化的结果
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public static int process(char[] str, int i) {
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if (i == str.length) {
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return 1;
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}
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if (str[i] == '0') {
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return 0;
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}
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if (str[i] == '1') {
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int res = process(str, i + 1);
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if (i + 1 < str.length) {
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res += process(str, i + 2);
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}
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return res;
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}
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if (str[i] == '2') {
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int res = process(str, i + 1);
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if (i + 1 < str.length && (str[i + 1] >= '0' && str[i + 1] <= '6')) {
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res += process(str, i + 2); // (i和i+1)作为单独的部分，后续有多少种方法
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}
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return res;
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}
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return process(str, i + 1);
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}
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public static int dp(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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public static void main(String[] args) {
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System.out.println(number("2132082"));
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System.out.println(dp("2132082"));
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}
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}
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