feat: Add python3 solution for problem 31 (azl397985856#115)

* Fix solution on problem 88 with js, fixs azl397985856#62

* Fix solution on problem 88, fixs azl397985856#62

* Add python3 solution for problem 31

problems/31.next-permutation.md

@@ -101,6 +101,37 @@ var nextPermutation = function(nums) {
   reverseRange(nums, i + 1, nums.length - 1);
 };
 ```
+Python3 Code:
+```python
+class Solution:
+    def nextPermutation(self, nums):
+        """
+        Do not return anything, modify nums in-place instead.
+        :param list nums
+        """
+        # 第一步，从后往前，找到下降点
+        down_index = None
+        for i in range(len(nums)-2, -1, -1):
+            if nums[i] < nums[i+1]:
+                down_index = i
+                break
+        # 如果没有下降点，重新排列
+        if down_index is None:
+            nums.reverse()
+        # 如果有下降点
+        else:
+            # 第二步，从后往前，找到比下降点大的数，对换位置
+            for i in range(len(nums)-1, i, -1):
+                if nums[down_index] < nums[i]:
+                    nums[down_index], nums[i] = nums[i], nums[down_index]
+                    break
+            # 第三部，重新排列下降点之后的数
+            i, j = down_index+1, len(nums)-1
+            while i < j:
+                nums[i], nums[j] = nums[j], nums[i]
+                i += 1
+                j -= 1
+```
 
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