PAT1051:Pop Sequence
1051. Pop Sequence (25)
Given a stack which can keep M numbers at most. Push N numbers in the order of 1, 2, 3, ..., N and pop randomly. You are supposed to tell if a given sequence of numbers is a possible pop sequence of the stack. For example, if M is 5 and N is 7, we can obtain 1, 2, 3, 4, 5, 6, 7 from the stack, but not 3, 2, 1, 7, 5, 6, 4.
Input Specification:
Each input file contains one test case. For each case, the first line contains 3 numbers (all no more than 1000): M (the maximum capacity of the stack), N (the length of push sequence), and K (the number of pop sequences to be checked). Then K lines follow, each contains a pop sequence of N numbers. All the numbers in a line are separated by a space.
Output Specification:
For each pop sequence, print in one line "YES" if it is indeed a possible pop sequence of the stack, or "NO" if not.
Sample Input:5 7 5 1 2 3 4 5 6 7 3 2 1 7 5 6 4 7 6 5 4 3 2 1 5 6 4 3 7 2 1 1 7 6 5 4 3 2Sample Output:
YES NO NO YES NO
思路
栈的应用。这里直接用vector模拟栈,不得不说vector太强大了,既可以当栈又可以当队列用。
这道题需要理清楚弹出序列不满足的条件:
1.压进去数字后的栈容量大于限定的容量。
2.弹出序列需求的弹出元素不在栈顶。
简单过程:
1.用一个bool值isSuccess来标识弹出序列是否可行,默认为true。
2.保存第一次压入操作的最大元素cur,然后遍历弹出序列:
1)如果弹出序列的当前元素temp比当前最大元素cur小,则它必须等于此时的栈顶元素top,否则要想得到和temp一样的值,就得不停弹出top,直到top == temp,但显然这样已经和弹出序列不匹配了,不可行,isSuccess置为false。
2)如果temp比最大元素cur更大,则把cur + 1和temp之间的元素压入栈中,并检查此时栈容量是否超过要求,超过表明该序列也不可行,isSuccess置为false。
3.遍历完弹出序列后,根据标识isSuccess的值输出YES or No即可。
代码
#include<iostream> #include<vector> using namespace std; int main() { int N,M,K; while(cin >> M >> N >> K) { vector<int> stk; while(K--) { bool isSuccess = true; int curmax = 1; stk.push_back(1); for(int i = 1; i <= N;i++) { int temp; cin >> temp; if(temp > curmax) { for(int j = curmax + 1; j <= temp;j++) stk.push_back(j); if(stk.size() > M) { isSuccess = false; } curmax = temp; } else { if(stk.back() != temp) { isSuccess = false; } } stk.pop_back(); } if(isSuccess) cout << "YES" << endl; else cout << "NO" << endl; } } }