0091. Decode Ways

Author: coderbean

markdown/0091. Decode Ways.md

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+### [91\. Decode Ways](https://leetcode.com/problems/decode-ways/)
+
+Difficulty: **Medium**
+
+
+A message containing letters from `A-Z` is being encoded to numbers using the following mapping:
+
+```
+'A' -> 1
+'B' -> 2
+...
+'Z' -> 26
+```
+
+Given a **non-empty** string containing only digits, determine the total number of ways to decode it.
+
+**Example 1:**
+
+```
+Input: "12"
+Output: 2
+Explanation: It could be decoded as "AB" (1 2) or "L" (12).
+```
+
+**Example 2:**
+
+```
+Input: "226"
+Output: 3
+Explanation: It could be decoded as "BZ" (2 26), "VF" (22 6), or "BBF" (2 2 6).
+```
+
+
+#### Solution A （推荐）
+
+Language: **Java**
+
+```java
+class Solution {
+    public int numDecodings(String s) { // 动态规划
+        if (s == null || s.length() == 0) {
+            return 0;
+        }
+        int[] dp = new int[s.length() + 1];
+        dp[0] = 1;
+        dp[1] = s.charAt(0) != '0' ? 1 : 0;
+        for (int i = 2; i <= s.length(); i++) {
+            int first = Integer.parseInt(s.substring(i - 1, i));
+            int second = Integer.parseInt(s.substring(i - 2, i));
+            if (first > 0 && first < 10) {
+                dp[i] += dp[i - 1];
+            }
+            if (second >= 10 && second <= 26) {
+                dp[i] += dp[i - 2];
+            }
+        }
+        return dp[s.length()];
+    }
+}
+```
+
+
+#### Solution B
+
+Language: **Java**
+
+```java
+class Solution {
+    public int numDecodings(String s) { // 动态规划
+        if (s.startsWith("0")) {
+            return 0;
+        }
+        return this.numDecodings(s.toCharArray(), s.length(), new int[s.length()]);
+    }
+
+    private int numDecodings(char[] chars, int length, int[] dp) {
+        if (length <= 1) {
+            return 1;
+        }
+        if (dp[length - 1] > 0) {
+            return dp[length - 1];
+        }
+        int num = 0;
+        if (chars[length - 1] != '0') {
+            num += numDecodings(chars, length - 1, dp);
+        } else {
+            if (chars[length - 2] == '0') {
+                dp[length - 1] = num;
+                return num;
+            }
+        }
+        if (chars[length - 2] != '0' && (chars[length - 2] - '0') * 10 + chars[length - 1] - '0' <= 26) {
+            num += numDecodings(chars, length - 2, dp);
+        }
+        dp[length - 1] = num;
+        return num;
+    }
+}
+```
+![pic](https://raw.githubusercontent.com/PicGoBed/PicBed/master/2019-08-03-tX1fHU.jpg)

src/main/java/leetcode/_91_/Main.java

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+package leetcode._91_;
+
+/**
+ * Created by zhangbo54 on 2019-03-04.
+ */
+public class Main {
+    public static void main(String[] args) {
+        Solution2 solution = new Solution2();
+        System.out.println( solution.numDecodings("101"));
+    }
+}
+

src/main/java/leetcode/_91_/Solution.java

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+package leetcode._91_;
+
+class Solution {
+    public int numDecodings(String s) { // 动态规划
+        if (s.startsWith("0")) {
+            return 0;
+        }
+        return this.numDecodings(s.toCharArray(), s.length(), new int[s.length()]);
+    }
+
+    private int numDecodings(char[] chars, int length, int[] dp) {
+        if (length <= 1) {
+            return 1;
+        }
+        if (dp[length - 1] > 0) {
+            return dp[length - 1];
+        }
+        int num = 0;
+        if (chars[length - 1] != '0') {
+            num += numDecodings(chars, length - 1, dp);
+        } else {
+            if (chars[length - 2] == '0') {
+                dp[length - 1] = num;
+                return num;
+            }
+        }
+        if (chars[length - 2] != '0' && (chars[length - 2] - '0') * 10 + chars[length - 1] - '0' <= 26) {
+            num += numDecodings(chars, length - 2, dp);
+        }
+        dp[length - 1] = num;
+        return num;
+    }
+}

src/main/java/leetcode/_91_/Solution2.java

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+package leetcode._91_;
+
+class Solution2 {
+    public int numDecodings(String s) { // 动态规划
+        if (s == null || s.length() == 0) {
+            return 0;
+        }
+        int[] dp = new int[s.length() + 1];
+        dp[0] = 1;
+        dp[1] = s.charAt(0) != '0' ? 1 : 0;
+        for (int i = 2; i <= s.length(); i++) {
+            int first = Integer.parseInt(s.substring(i - 1, i));
+            int second = Integer.parseInt(s.substring(i - 2, i));
+            if (first > 0 && first < 10) {
+                dp[i] += dp[i - 1];
+            }
+            if (second >= 10 && second <= 26) {
+                dp[i] += dp[i - 2];
+            }
+        }
+        return dp[s.length()];
+    }
+}

src/main/java/leetcode/_91_/solution.md

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+### [91\. Decode Ways](https://leetcode.com/problems/decode-ways/)
+
+Difficulty: **Medium**
+
+
+A message containing letters from `A-Z` is being encoded to numbers using the following mapping:
+
+```
+'A' -> 1
+'B' -> 2
+...
+'Z' -> 26
+```
+
+Given a **non-empty** string containing only digits, determine the total number of ways to decode it.
+
+**Example 1:**
+
+```
+Input: "12"
+Output: 2
+Explanation: It could be decoded as "AB" (1 2) or "L" (12).
+```
+
+**Example 2:**
+
+```
+Input: "226"
+Output: 3
+Explanation: It could be decoded as "BZ" (2 26), "VF" (22 6), or "BBF" (2 2 6).
+```
+
+
+#### Solution A （推荐）
+
+Language: **Java**
+
+```java
+class Solution {
+    public int numDecodings(String s) { // 动态规划
+        if (s == null || s.length() == 0) {
+            return 0;
+        }
+        int[] dp = new int[s.length() + 1];
+        dp[0] = 1;
+        dp[1] = s.charAt(0) != '0' ? 1 : 0;
+        for (int i = 2; i <= s.length(); i++) {
+            int first = Integer.parseInt(s.substring(i - 1, i));
+            int second = Integer.parseInt(s.substring(i - 2, i));
+            if (first > 0 && first < 10) {
+                dp[i] += dp[i - 1];
+            }
+            if (second >= 10 && second <= 26) {
+                dp[i] += dp[i - 2];
+            }
+        }
+        return dp[s.length()];
+    }
+}
+```
+
+
+#### Solution B
+
+Language: **Java**
+
+```java
+class Solution {
+    public int numDecodings(String s) { // 动态规划
+        if (s.startsWith("0")) {
+            return 0;
+        }
+        return this.numDecodings(s.toCharArray(), s.length(), new int[s.length()]);
+    }
+
+    private int numDecodings(char[] chars, int length, int[] dp) {
+        if (length <= 1) {
+            return 1;
+        }
+        if (dp[length - 1] > 0) {
+            return dp[length - 1];
+        }
+        int num = 0;
+        if (chars[length - 1] != '0') {
+            num += numDecodings(chars, length - 1, dp);
+        } else {
+            if (chars[length - 2] == '0') {
+                dp[length - 1] = num;
+                return num;
+            }
+        }
+        if (chars[length - 2] != '0' && (chars[length - 2] - '0') * 10 + chars[length - 1] - '0' <= 26) {
+            num += numDecodings(chars, length - 2, dp);
+        }
+        dp[length - 1] = num;
+        return num;
+    }
+}
+```
+![pic](https://raw.githubusercontent.com/PicGoBed/PicBed/master/2019-08-03-tX1fHU.jpg)