題目描述
用兩個棧實現一個隊列。隊列的聲明如下,請實現它的兩個函數 appendTail 和 deleteHead ,分別完成在隊列尾部插入整數和在隊列頭部刪除整數的功能。(若隊列中沒有元素,deleteHead 操作返回 -1 )
示例 1:
輸入:
[“CQueue”,“appendTail”,“deleteHead”,“deleteHead”]
[[],[3],[],[]]
輸出:[null,null,3,-1]
示例 2:
輸入:
[“CQueue”,“deleteHead”,“appendTail”,“appendTail”,“deleteHead”,“deleteHead”]
[[],[],[5],[2],[],[]]
輸出:[null,-1,null,null,5,2]
提示:
1 <= values <= 10000
最多會對 appendTail、deleteHead 進行 10000 次調用
思路一
用棧s1作爲主棧,棧s2爲輔助棧。每次隊列增加元素,將s1每個元素壓棧到s2,再將新增元素壓入s1棧底,最後將s2元素全部壓回s1
該方法不是最優解,時間複雜度較高
class CQueue {
private:
stack<int> s1;
stack<int> s2;
public:
CQueue() {
}
void appendTail(int value) {
while(!s1.empty())
{
s2.push(s1.top());
s1.pop();
}
s1.push(value);
while(!s2.empty())
{
s1.push(s2.top());
s2.pop();
}
}
int deleteHead() {
if(s1.empty()) return -1;
int x = s1.top();
s1.pop();
return x;
}
};
/**
* Your CQueue object will be instantiated and called as such:
* CQueue* obj = new CQueue();
* obj->appendTail(value);
* int param_2 = obj->deleteHead();
*/
思路二(最優解)
棧s1、s2各司其職,s1用來隊列頭部增加元素,s2用來隊列尾部刪減元素。只需當s2爲空時,將s1的所有元素壓入s2即可.
class CQueue {
stack<int>s1;
stack<int>s2;
public:
CQueue() {
}
void appendTail(int value) {
s1.push(value);
}
int deleteHead() {
if(s2.empty()){
if(s1.empty()){
return -1;
}
else{
while(!s1.empty()){
s2.push(s1.top());
s1.pop();
}
}
}
int head=s2.top();
s2.pop();
return head;
}
};
/**
* Your CQueue object will be instantiated and called as such:
* CQueue* obj = new CQueue();
* obj->appendTail(value);
* int param_2 = obj->deleteHead();
*/