PAT甲级 1167 Cartesian Tree（30）

思路：碰到层序遍历就想到队列，也就是在区间中，先找到最小值（根结点）下标，入队。出队时，输出。然后分别找左右子树中的根节点入队，以此类推。另外需要维护一个visited数组，作为遍历找根节点时的结束位置。

#include <bits/stdc++.h>
using namespace std;

int N;
vector<int> inorder;
vector<int> visited;
queue<int> q;

int search(int i, int way) {
	int minIndex = -1, minNum = INT_MAX;
	if (way == 0) {
		//左
		while (i >= 0 && visited[i] == 0) {
			if (inorder[i] < minNum) {
				minIndex = i;
				minNum = inorder[i];
			}
			i--;
		}
	} else {
		//右 
		while (i < N && visited[i] == 0) {
			if (inorder[i] < minNum) {
				minIndex = i;
				minNum = inorder[i];
			}
			i++;
		}
	}
	return minIndex;
}

int main() {
	cin >> N;
	inorder.resize(N);
	visited.resize(N, 0);
	int minIndex = -1, minNum = INT_MAX;
	for (int i = 0; i < N; ++i) {
		cin >> inorder[i];
		if (inorder[i] < minNum) {
			minIndex = i;
			minNum = inorder[i];
		}
	}
	q.push(minIndex);
	visited[minIndex] = 1;
	while (true) {
		int front = q.front();
		q.pop();
		cout << inorder[front];
		minIndex = search(front - 1, 0);
		if (minIndex != -1) {
			visited[minIndex] = 1;
			q.push(minIndex);
		}
		minIndex = search(front + 1, 1);
		if (minIndex != -1) {
			visited[minIndex] = 1;
			q.push(minIndex);
		}
		if (!q.empty()) {
			cout << " ";
		} else {
			cout << endl;
			break;
		}
	}
	
	return 0;
}