Merge pull request #1071 from hutbzc/master

添加（0139.单词拆分.md）：增加Java回溯+记忆化版本；添加（0045.跳跃游戏II.md）：补充Java版本2；修改（0376.摆动序列.md）：修改了逻辑小错误

problems/0045.跳跃游戏II.md

@@ -142,6 +142,7 @@ public:
 
 ### Java 
 ```Java
+// 版本一
 class Solution {
     public int jump(int[] nums) {
         if (nums == null || nums.length == 0 || nums.length == 1) {
@@ -172,7 +173,30 @@ class Solution {
 }
 ```
 
+```java
+// 版本二
+class Solution {
+    public int jump(int[] nums) {
+        int result = 0;
+        // 当前覆盖的最远距离下标
+        int end = 0;
+        // 下一步覆盖的最远距离下标
+        int temp = 0;
+        for (int i = 0; i <= end && end < nums.length - 1; ++i) {
+            temp = Math.max(temp, i + nums[i]);
+            // 可达位置的改变次数就是跳跃次数
+            if (i == end) {
+                end = temp;
+                result++;
+            }
+        }
+        return result;
+    }
+}
+```
+
 ### Python
+
 ```python
 class Solution:
     def jump(self, nums: List[int]) -> int:

problems/0139.单词拆分.md

@@ -248,6 +248,36 @@ class Solution {
         return valid[s.length()];
     }
 }
+
+// 回溯法+记忆化
+class Solution {
+    public boolean wordBreak(String s, List<String> wordDict) {
+        Set<String> wordDictSet = new HashSet(wordDict);
+        int[] memory = new int[s.length()];
+        return backTrack(s, wordDictSet, 0, memory);
+    }
+    
+    public boolean backTrack(String s, Set<String> wordDictSet, int startIndex, int[] memory) {
+        // 结束条件
+        if (startIndex >= s.length()) {
+            return true;
+        }
+        if (memory[startIndex] != 0) {
+            // 此处认为：memory[i] = 1 表示可以拼出i 及以后的字符子串， memory[i] = -1 表示不能
+            return memory[startIndex] == 1 ? true : false;
+        }
+        for (int i = startIndex; i < s.length(); ++i) {
+            // 处理 递归 回溯       循环不变量：[startIndex, i + 1)
+            String word = s.substring(startIndex, i + 1);
+            if (wordDictSet.contains(word) && backTrack(s, wordDictSet, i + 1, memory)) {
+                memory[startIndex] = 1;
+                return true;
+            }
+        }
+        memory[startIndex] = -1;
+        return false;
+    }
+}
 ```
 
 Python：

problems/0376.摆动序列.md

@@ -174,7 +174,7 @@ public:
 ```Java
 class Solution {
     public int wiggleMaxLength(int[] nums) {
-        if (nums == null || nums.length <= 1) {
+        if (nums.length <= 1) {
             return nums.length;
         }
         //当前差值