二、算法分类经典题目练习：栈、队列、堆
225.用队列实现栈
临时队列
class MyStack {
public:
/** Initialize your data structure here. */
MyStack() {
}
/** Push element x onto stack. */
void push(int x) {
queue<int> temp_queue;
temp_queue.push(x);
while(!q.empty()){
temp_queue.push(q.front());
q.pop();
}
while(!temp_queue.empty()){
q.push(temp_queue.front());
temp_queue.pop();
}
}
/** Removes the element on top of the stack and returns that element. */
int pop() {
int res = q.front();
q.pop();
return res;
}
/** Get the top element. */
int top() {
return q.front();
}
/** Returns whether the stack is empty. */
bool empty() {
return q.empty();
}
private:
queue<int> q;
};
/**
* Your MyStack object will be instantiated and called as such:
* MyStack* obj = new MyStack();
* obj->push(x);
* int param_2 = obj->pop();
* int param_3 = obj->top();
* bool param_4 = obj->empty();
*/
232.用栈实现队列
临时栈
class MyQueue {
public:
/** Initialize your data structure here. */
MyQueue() {
}
/** Push element x to the back of queue. */
void push(int x) {
stack<int> temp_stack;
while(!s.empty()){
temp_stack.push(s.top());
s.pop();
}
s.push(x);
while(!temp_stack.empty()){
s.push(temp_stack.top());
temp_stack.pop();
}
}
/** Removes the element from in front of queue and returns that element. */
int pop() {
int res = s.top();
s.pop();
return res;
}
/** Get the front element. */
int peek() {
return s.top();
}
/** Returns whether the queue is empty. */
bool empty() {
return s.empty();
}
private:
stack<int> s;
};
/**
* Your MyQueue object will be instantiated and called as such:
* MyQueue* obj = new MyQueue();
* obj->push(x);
* int param_2 = obj->pop();
* int param_3 = obj->peek();
* bool param_4 = obj->empty();
*/
155.最小栈
class MinStack {
public:
/** initialize your data structure here. */
MinStack() {
}
void push(int x) {
s.push(x);
if(!min.empty()){
x = min.top() < x ? min.top() : x;
min.push(x);
}else{
min.push(x);
}
}
void pop() {
s.pop();
min.pop();
}
int top() {
return s.top();
}
int getMin() {
return min.top();
}
private:
stack<int> s;
stack<int> min;
};
/**
* Your MinStack object will be instantiated and called as such:
* MinStack* obj = new MinStack();
* obj->push(x);
* obj->pop();
* int param_3 = obj->top();
* int param_4 = obj->getMin();
*/
合法的出栈顺序
http://poj.org/problem?id=1363
#include 224.基本计算器
class Solution {
public:
int calculate(string s) {
stack<int> st;
int n = s.size(), res = 0, sign = 1;
for(int i = 0; i < n; ++i){
int num = 0;
if(s[i]>='0'){
while(i<n&&s[i]>='0'){
num = num * 10 + (s[i] - '0');
i++;
}
i--;
res += num * sign;
}
else if(s[i]=='+') sign = 1;
else if(s[i]=='-') sign = -1;
else if(s[i]=='('){
st.push(res);
st.push(sign);
res = 0;
sign = 1;
}
else if(s[i]==')'){
res *= st.top(); st.pop();
res += st.top(); st.pop();
}
}
return res;
}
};
215.数组中的第K个最大的元素
优先队列
class Solution {
public:
int findKthLargest(vector<int>& nums, int k) {
priority_queue<int, vector<int>, greater<int>> q;
for(int i = 0; i < nums.size(); ++i){
if(q.size() < k){
q.push(nums[i]);
}else if(q.top() < nums[i]){
q.pop();
q.push(nums[i]);
}
}
return q.top();
}
};
295.数据流的中位数
大顶堆小顶堆
class MedianFinder {
public:
/** initialize your data structure here. */
MedianFinder() {
}
void addNum(int num) {
if(big_queue.empty()){
big_queue.push(num);
return;
}
if(big_queue.size()==small_queue.size()){
if(num < big_queue.top()){
big_queue.push(num);
}else{
small_queue.push(num);
}
}else if(big_queue.size() > small_queue.size()){
if(num < big_queue.top()){
int tmp = big_queue.top();
big_queue.pop();
small_queue.push(tmp);
big_queue.push(num);
}else{
small_queue.push(num);
}
}else{
if(num > small_queue.top()){
int tmp = small_queue.top();
small_queue.pop();
small_queue.push(num);
big_queue.push(tmp);
}else{
big_queue.push(num);
}
}
}
double findMedian() {
if(big_queue.size()==small_queue.size()){
return (double)(big_queue.top()+small_queue.top())/2;
}else if(big_queue.size()>small_queue.size()){
return big_queue.top();
}else{
return small_queue.top();
}
}
private:
priority_queue<int, vector<int>, greater<int>> small_queue;
priority_queue<int, vector<int>, less<int>> big_queue;
};
/**
* Your MedianFinder object will be instantiated and called as such:
* MedianFinder* obj = new MedianFinder();
* obj->addNum(num);
* double param_2 = obj->findMedian();
*/