一、递归（Recursion）

定义：函数直接或者间接调用自己

二、力扣509题 

class Solution(object):
    def fib(self, n):
        """
        :type n: int
        :rtype: int
        """
        if n < 2:
            return 0 if n == 0 else 1
        sum = self.fib(n-1) + self.fib(n-2)
        return sum

三、力扣206题

class Solution(object):
    def reverseList(self, head):
        """
        :type head: ListNode
        :rtype: ListNode
        """
        if head is None or head.next is None:
            return head
        p = self.reverseList(head.next)
        head.next.next = head
        head.next = None
        return p

 四、力扣344题

递归法：

class Solution(object):
    def reverseString(self, s):
        """
        :type s: List[str]
        :rtype: None Do not return anything, modify s in-place instead.
        """
        self.recursion(s,0,len(s)-1)
    def recursion(self,s,left,right):
        if left >= right:
            return
        self.recursion(s,left+1,right-1)
        s[left],s[right] = s[right],s[left]

双指针法：

class Solution(object):
    def reverseString(self, s):
        """
        :type s: List[str]
        :rtype: None Do not return anything, modify s in-place instead.
        """
        left = 0
        right = len(s) - 1
        while(left <= right):
            s[left],s[right] = s[right],s[left]
            left += 1
            right -= 1
        return