20.有效的括号

判断括号是否配对

class Solution {
    public boolean isValid(String s) {
        Deque<Character> deque = new LinkedList<>();
        char ch;
        for(int i = 0; i < s.length(); i++){
            ch = s.charAt(i);
            if(ch == '('){
                deque.push(')');
            }else if(ch == '{'){
                deque.push('}');
            }else if(ch == '['){
                deque.push(']');
            }else if(deque.isEmpty() || ch != deque.peek()){
                return false;
            }else{
                deque.pop();
            }
        }
        return deque.isEmpty();
    }
}

1047.删除字符串相邻重复项

输入： "abbaca"
输出： "ca"

class Solution {
    public String removeDuplicates(String s) {
        ArrayDeque<Character> deque = new ArrayDeque<>();
        char ch;
        for(int i = 0; i < s.length(); i++){
            ch = s.charAt(i);
            if(deque.isEmpty() || ch != deque.peek()){
                deque.push(ch);
            }else{
                deque.pop();
            }
        }
        String str = "";
        while(!deque.isEmpty()){
            str = deque.pop() + str;
        }
        return str;
    }
}

150.逆波兰表达式

输入： tokens = ["2","1","+","3","*"]
输出： 9

class Solution {
    public int evalRPN(String[] tokens) {
        Deque<Integer> stack = new LinkedList();
        for(String s : tokens){
            if("+".equals(s)){
                stack.push(stack.pop() + stack.pop());
            }else if("-".equals(s)){
                stack.push(-stack.pop() + stack.pop());//谁是被减数不要搞错
            }else if("*".equals(s)){
                stack.push(stack.pop() * stack.pop());
            }else if("/".equals(s)){
                int temp1 = stack.pop();
                int temp2 = stack.pop();
                stack.push(temp2 / temp1);
            }else{
                stack.push(Integer.valueOf(s));
            }
        }
        return stack.pop();
    }
}