1.求root(N,k)
https://www.nowcoder.com/practice/9324a1458c564c4b9c4bfc3867a2aa66?tpId=40&tqId=21347&tPage=1&rp=1&ru=/ta/kaoyan&qru=/ta/kaoyan/question-ranking
2.最小花费
https://www.nowcoder.com/practice/e6df3e3005e34e2598b9b565cfe797c9?tpId=40&tqId=21354&tPage=2&rp=2&ru=/ta/kaoyan&qru=/ta/kaoyan/question-ranking
