PAT 1110 Complete Binary Tree

Given a tree, you are supposed to tell if it is a complete binary tree.

Input Specification:
Each input file contains one test case. For each case, the first line gives a positive integer N (≤20) which is the total number of nodes in the tree -- and hence the nodes are numbered from 0 to N−1. Then N lines follow, each corresponds to a node, and gives the indices of the left and right children of the node. If the child does not exist, a - will be put at the position. Any pair of children are separated by a space.

Output Specification:
For each case, print in one line YES and the index of the last node if the tree is a complete binary tree, or NO and the index of the root if not. There must be exactly one space separating the word and the number.

Sample Input 1:

9
7 8
- -
- -
- -
0 1
2 3
4 5
- -
- -

Sample Output 1:
YES 8

Sample Input 2:

8
- -
4 5
0 6
- -
2 3
- 7
- -
- -

Sample Output 2:
NO 1

#include<iostream>
#include<vector>
#include<string>
using namespace std;
vector<int> tree(21,-1), Left(20,-1), Right(20,-1), record(20,0);
int n;
void buildtree(int root){ 
    if(Left[tree[root]]!=-1){
    	if(2*root+1>19)
    		return;	
      tree[2*root+1]=Left[tree[root]];
      buildtree(2*root+1);
    }
    if(Right[tree[root]]!=-1){
    	if(2*root+2>19)
        	return;
      tree[2*root+2]=Right[tree[root]];
      buildtree(2*root+2);
    }
}
int main(){
  cin>>n;
  string l, r;
  for(int i=0; i<n; i++){
    cin>>l>>r;
    if(l[0]!='-'){
      Left[i]=stoi(l);
      record[Left[i]]=1;
    }
    if(r[0]!='-'){
      Right[i]=stoi(r);
      record[Right[i]]=1;
    }
  }
  int i=0;
  for(; i<n; i++)
     if(record[i]==0)
        break;
  tree[0]=i;
  buildtree(0);
  int cnt=0;
  for(int i=0; i<=20; i++)
  	if(tree[i]==-1)
  	  if(i==n){
  	  		cout<<"YES "<<tree[n-1]<<endl;
  	  		return 0;
		}
      else{
      		cout<<"NO "<<tree[0]<<endl;
      		return 0; 
	  } 
}