That's just what I was thinking.
 →  같은생각이야.

[예문]
Jeehee : Minho is late again. I don't want to miss the concert.  
          민호는또늦나봐. 난 그콘세트놓치고싶지않아.

Hosun: I can't wait any longer. I think we'd better go  without him.
        더이상기다릴수없어. 그냥가는게 어때?   

Jeehee:That's just what I was thinking. Let's go.
       같은생각이야. 가자.