4.栈

栈(stack)

栈是一种特殊的线性表,只能在一端进行操作(后进先出,Last In First Out,LIFO)

• 往栈中添加元素的操作,一般叫做push,入栈
• 从栈中移除元素的操作,一般叫做pop,出栈(只能移除栈顶元素,也叫做弹出栈顶元素)

注意,这里说的栈与内存中的栈空间是两个不同的概念

判断字符串里面的括号是否是成对出现

python实现

SYMBOLS = {'}': '{', ']': '[', ')': '(', '>': '<'}
SYMBOLS_L, SYMBOLS_R = SYMBOLS.values(), SYMBOLS.keys()
def check(s):
 stack = []
 for c in s:
  if c in SYMBOLS_L:
   # 左符号入栈
   stack.append(c)
  elif c in SYMBOLS_R:
   # 右符号要么出栈，要么匹配失败
   if stack and stack[-1] == SYMBOLS[c]:
    stack.pop()
   else:
    return False
 return True
print(check("3 * {3 +[(2 -3) * (4+5)]}"))
print(check("3 * {3+ [4 - 6}]"))

java实现

package zh.datastructures.stack;

import java.util.Stack;

public class Parentheses {
    public boolean isValid(String s) {
        Stack<Character> stack = new Stack<>();
        
        for (int i = 0; i < s.length(); i++) {
            char c = s.charAt(i);
            if (c=='(' || c=='{' ||c== '[') {
                stack.push(c);
            }else {
                if (stack.isEmpty()) return false;
                char left = stack.pop();
                if (left == '(' && c != ')') return false;
                if (left == '[' && c != ']') return false;
                if (left == '{' && c != '}') return false;
            }
        }
        return stack.isEmpty();
    }
    
    public static void main(String[] args) {
        Parentheses test = new Parentheses();
        System.out.println(test.isValid("{}("));//false
        System.out.println(test.isValid("{}"));//true
    }
}