update

Author: jasoncao11

P106. 寻找峰值.md

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+***峰值元素是指其值严格大于左右相邻值的元素。给你一个整数数组 nums，找到峰值元素并返回其索引。数组可能包含多个峰值，在这种情况下，返回 任何一个峰值 所在位置即可。***
+
+```
+输入：nums = [1,2,3,1]
+输出：2
+解释：3 是峰值元素，你的函数应该返回其索引 2。
+```
+
+```
+class Solution:
+    def findPeakElement(self, nums: List[int]) -> int:
+        i = 0
+        j = len(nums)-1
+        while i < j:
+            mid = (i+j)//2
+            if nums[mid] > nums[mid+1]:
+                j = mid
+            else:
+                i = mid+1
+        return i 
+```

P107. 四数相加 II.md

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+```
+给你四个整数数组 nums1、nums2、nums3 和 nums4 ，数组长度都是 n ，请你计算有多少个元组 (i, j, k, l) 能满足：
+
+0 <= i, j, k, l < n
+nums1[i] + nums2[j] + nums3[k] + nums4[l] == 0
+```
+
+```
+class Solution:
+    def fourSumCount(self, nums1: List[int], nums2: List[int], nums3: List[int], nums4: List[int]) -> int:
+        cnt = 0
+        countAB = collections.Counter([a+b for a in nums1 for b in nums2])
+        for c in nums3:
+            for d in nums4:
+                countCD = c+d
+                if -countCD in countAB:
+                    cnt += countAB[-countCD]
+        return cnt
+```

P108. 奇偶链表.md

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+![algo45](./images/algo45.jpg)
+
+```
+# Definition for singly-linked list.
+# class ListNode:
+#     def __init__(self, val=0, next=None):
+#         self.val = val
+#         self.next = next
+class Solution:
+    def oddEvenList(self, head: Optional[ListNode]) -> Optional[ListNode]:
+        if not head:
+            return None
+        oddhead = head
+        evenhead = head.next
+        odd, even = oddhead, evenhead
+        while even and even.next:
+            odd.next = even.next
+            odd = odd.next
+            even.next = odd.next
+            even = even.next
+        odd.next = evenhead
+        return oddhead
+```