代码随想录算法训练营第10天 | 复习队列和栈
2024年7月12日
题232. 用栈实现队列
两边倒即可,要出队列就倒到右边去,然后再回来。
class MyQueue {
Stack<Integer> s1;
Stack<Integer> s2;
int size;
public MyQueue() {
s1 = new Stack<>();
s2 = new Stack<>();
size= 0;
}
public void push(int x) {
s1.push(x);
size+=1;
}
public int pop() {
while(!s1.isEmpty()){
s2.push(s1.pop());
}
int res = s2.pop();
while(!s2.isEmpty()){
s1.push(s2.pop());
}
size-=1;
return res;
}
public int peek() {
while(!s1.isEmpty()){
s2.push(s1.pop());
}
int res = s2.peek();
while(!s2.isEmpty()){
s1.push(s2.pop());
}
return res;
}
public boolean empty() {
return size==0;
}
}
/**
* 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();
* boolean param_4 = obj.empty();
*/
题225. 用队列实现栈
复习了Java中Queue的实现类LinkedList,可以用offer从队尾添加元素,poll头部取出元素。
class MyStack {
Queue<Integer> q1;
Queue<Integer> q2;
int size;
public MyStack() {
q1 = new LinkedList<>();
q2 = new LinkedList<>();
size=0;
}
public void push(int x) {
q1.offer(x);
size+=1;
}
public int pop() {
int n = size;
while(n>1){
q2.offer(q1.poll());
n-=1;
}
int res = q1.poll();
size-=1;
while(!q2.isEmpty()){
q1.offer(q2.poll());
}
return res;
}
public int top() {
int n = size;;
while(n>1){
q2.offer(q1.poll());
n-=1;
}
int res = q1.poll();
q2.offer(res);
while(!q2.isEmpty()){
q1.offer(q2.poll());
}
return res;
}
public boolean empty() {
return size==0;
}
}
/**
* 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();
* boolean param_4 = obj.empty();
*/
题20. 有效的括号
用栈即可,要注意pop如果是空栈会直接报错,所以pop前要做好判断。
class Solution {
public boolean isValid(String s) {
Stack<Character> st = new Stack<>();
for(int i=0;i<s.length();i++){
char x = s.charAt(i);
if((x=='(')||(x=='{')||(x=='[')){
st.push(x);
}
if(x==')'){
if(st.isEmpty()){
return false;
}
char xx = st.pop();
if(xx!='('){
return false;
}
}
if(x=='}'){
if(st.isEmpty()){
return false;
}
char xx = st.pop();
if(xx!='{'){
return false;
}
}
if(x==']'){
if(st.isEmpty()){
return false;
}
char xx = st.pop();
if(xx!='['){
return false;
}
}
}
if(!st.isEmpty()){
return false;
}
return true;
}
}
题1047. 删除字符串中的所有相邻重复项
用栈,如果新来的char和栈顶相同,就把栈顶的去掉,否则入栈。
错误还是在忘了判断栈为空就pop,这样会直接报错。
class Solution {
public String removeDuplicates(String s) {
Stack<Character> stack = new Stack<>();
for(int i=0;i<s.length();i++){
char c = s.charAt(i);
if(stack.isEmpty()){
stack.push(c);
continue;
}
if(top(stack)==c){
stack.pop();
continue;
}else{
stack.push(c);
}
}
StringBuilder sb = new StringBuilder();
while(!stack.isEmpty()){
sb.append(stack.pop());
}
sb = sb.reverse();
return new String(sb);
}
public Character top(Stack<Character> stack){
char c = stack.pop();
stack.push(c);
return c;
}
}