Understanding recursion

The meaning of recursion

The Chinese understanding of recursion is transfer and regression , Passing means passing something between each level of recursion , Regression is to go back to the first recursion . Remember that a little recursive process is doing the same thing over and over again .

How to write recursion ？

A common template for recursion is ：① The termination condition of recursion , That is, when the recursion ends ;② What this round of recursion needs to do , That is, the processing of data , Including the value returned by the next level of recursion and the current round of value processing ;③ This recursion needs to return the value of the upper recursion .

example

Try your best to explain the topic , The title is as follows ：

Recursion is always processed up from the end of recursion . So its code is ：/**

• Definition for singly-linked list.
• struct ListNode {

• int val;
  

• ListNode *next;
  

• ListNode(int x) : val(x), next(NULL) {}
  

• };
  /
  // recursive
  class Solution {
  public:
  ListNode swapPairs(ListNode* head) {
  // Recursive termination condition
  if(headnullptr||head->nextnullptr)
  return head;
  // What this level recursion needs to do
  ListNode *node=swapPairs(head->next->next);
  ListNode *temp=head->next;
  head->next=node;
  temp->next=head;
  // Return the value of upper level recursion
  return temp;
  }
  };