20. 有效的括号

 public boolean isValid(String s) {
        // 自己的做法,遇见'{' 入'{'
        Stack<Character> stack = new Stack<>();
        for (int i = 0; i < s.length(); i++) {
            char ch = s.charAt(i);
            if ((ch == '[' || ch == '{') || ch == '(') {
                stack.push(ch);
            } else if (!stack.isEmpty()) {
                Character pop = stack.pop();
                if (pop == '[' && ch == ']') {
                    continue;
                } else if (pop == '{' && ch == '}') {
                    continue;
                } else if (pop == '(' && ch == ')') {
                    continue;
                }
                return false;
            } else {
                return false;
            }
        }
        if (!stack.isEmpty()) {
            return false;
        }
        return true;
    }
//==========================================================上面是自己的方法
// =========================================================-=下面是看了随想录后写的方法
    public boolean isValid2(String s) {
        Stack<Character> stack = new Stack<>();
        for (int i = 0; i < s.length(); i++) {
            char ch = s.charAt(i);
            if (ch == '(') {
                stack.push(')');
            } else if (ch == '{') {
                stack.push('}');
            } else if (ch == '[') {
                stack.push(']');
            } else if (stack.isEmpty()) {
                return false;
            } else {
                if (ch != stack.pop()) {
                    return false;
                }
            }
        }
        return stack.isEmpty();
    }

1047. 删除字符串中的所有相邻重复项

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

150. 逆波兰表达式求值

public int evalRPN(String[] tokens) {
        ArrayDeque<String> stack = new ArrayDeque<String>();
        for (String token : tokens) {
            if (isCal(token)) {
                int var1 = Integer.parseInt(stack.pop());
                int var2 = Integer.parseInt(stack.pop());
                String var3 = null;
                switch (token) {
                //===================唯一 要注意的地方是 ,下面这个 var2 和 var1 的放置顺序
                    case "+":
                        var3 = String.valueOf(var2 + var1);
                        break;
                    case "-":
                        var3 = String.valueOf(var2 - var1);
                        break;
                    case "*":
                        var3 = String.valueOf(var2 * var1);
                        break;
                    case "/":
                        var3 = String.valueOf(var2 / var1);
                        break;
                }
                stack.push(var3);
                continue;
            }
            stack.push(token);
        }
        assert stack.peek() != null;
        return Integer.parseInt(stack.peek());
    }

    //====================小小的辅助函数
	public boolean isCal(String str) {
        return "+".equals(str) || "-".equals(str) || "*".equals(str) || "/".equals(str);
    }