React中useCallbackuseMemo到底该怎么用

 import React, { useState, useCallback, useEffect } from 'react';
 const set = new Set();
 export default function Parent() {
   const [count, setCount] = useState(1);
   const callback = useCallback(() => {
     console.log(count);
     return count
   }, [count]); //count更新时执行
   return (
   <div>
     <h4>parent count:{count}</h4>
     <div>
       <button onClick={() => setCount(count + 1)}>+</button>
     </div>
     <Child callback={callback} />
   </div>
   )
 }
 const Child = ({callback}) =>{
   const [count,setCount] = useState(0);
   useEffect(()=>{
     setCount(callback())
   },[callback]) //当callback更新时执行callback函数，得到parent组件最新的count
   return <child>
     count:{count}
   </child>
 } 

 import React, { useState, useMemo } from 'react';
 export default function Memos() {
   const [count, setCount] = useState(1);
   const memo = useMemo(() => {
     console.log("count更新了"+count);
     return count
   }, [count]); //count更新时执行
   return <div>
     <h4>{count}</h4>
     <div>
       <button onClick={() => setCount(count + 1)}>+</button>
     </div>
   </div>;
 }