普通栈

LeetCode20 有效的括号

LeetCode20 有效的括号

定义一个辅助map，判断字符串的字符是否在]})中。一旦是右括号就要弹出元素，判断匹配。

class Solution {public boolean isValid(String s) {if (s.length() % 2 == 1) {return false;}Map<Character, Character> pairs = new HashMap<>();pairs.put(')', '(');pairs.put(']', '[');pairs.put('}', '{');Stack<Character> stack = new Stack<>();for (int i = 0; i < s.length(); i++) {if (pairs.containsKey(s.charAt(i))) {if (stack.isEmpty() || pairs.get(s.charAt(i)) != stack.peek()) {return false;}stack.pop();} else {stack.push(s.charAt(i));}}if (!stack.isEmpty()) {return false;}return true;}
}

LeetCode155. 最小栈

LeetCode155. 最小栈

使用栈记录最小的元素。

class MinStack {private Stack<Integer> stack;private Stack<Integer> minStack;public MinStack() {stack = new Stack<>();minStack = new Stack<>();}public void push(int val) {stack.push(val);if (minStack.isEmpty() || minStack.peek() >= val) {minStack.push(val);}}public void pop() {Integer pop = stack.pop();if (pop.equals(minStack.peek())) {minStack.pop();}}public int top() {return stack.peek();}public int getMin() {return minStack.peek();}
}

LeetCode232. 用栈实现队列

LeetCode232. 用栈实现队列