LeetCode刷题day020 (Jieky)

LeetCode第20题

/*
Given a string containing just the characters ‘(’, ‘)’, ‘{’, ‘}’, ‘[’ and ‘]’, determine if the input string is valid.

An input string is valid if:

Open brackets must be closed by the same type of brackets. Open brackets must be closed in the correct order. Note that an empty string is also considered valid.

Example 1:
Input: "()"
Output: true

Example 2:
Input: "()[]{}"
Output: true

Example 3:
Input: "(]"
Output: false

Example 4:
Input: "([)]"
Output: false

Example 5:
Input: "{[]}"
Output: true
*/
import java.util.*;
public class ValidParentheses{
	public static void main(String[] args){
	String str = "{[]}";
		ValidParentheses vp = new ValidParentheses();
		boolean result = vp.isValid02(str);
		System.out.println(result);
	}
	
	// 使用Character 比 String 效率更高
	public boolean isValid02(String s){
		if(s == null) return false;
		
		if(s != null && s.length() == 0) return true;
		
		HashMap<Character,Character> pairs = new HashMap<Character,Character>(); 
		pairs.put(')','(');
		pairs.put(']','[');
		pairs.put('}','{');
		
		Stack<Character> elements = new Stack<Character>();
		
		// 将字符串转为字符数组
		char[] array = s.toCharArray();
		
		for (int i=0;i<s.length();i++){
			char temp = array[i];
			// 左括号入栈
			if (temp == '(' || temp == '{' || temp == '['){
				elements.push(temp);
			// 右括号校对栈顶元素是否匹配，匹配则栈顶元素出栈（包装类和基本类型可以自动转换）
			}else if(!elements.empty() && pairs.get(temp) == elements.peek()){
				elements.pop();
			// 当前右括号和栈顶元素不匹配，或栈为空且还有右括号
			}else{
				return false;
			}
		}
		
		// 查看栈是否为空，栈里面不可能有右括号，主要查看是否还有左括号
		if(elements.empty()){
			return true;
		}else{
			return false;
		}
	}
	
	public boolean isValid01(String s){
		if(s == null) return false;
		
		if(s != null && s.length() == 0) return true;
		
		HashMap<String,String> pairs = new HashMap<String,String>(); 
		pairs.put(")","(");
		pairs.put("}","{");
		pairs.put("]","[");
		
		Stack<String> elements = new Stack<String>();
		
		String temp = null;
		for (int i=0;i<s.length();i++){
			temp = s.substring(i,i+1);
			// 左括号入栈
			if (temp.equals("(") || temp.equals("{") || temp.equals("[")){
				elements.push(temp);
			// 右括号校对栈顶元素是否匹配，匹配则栈顶元素出栈
			}else if(!elements.empty() && pairs.get(temp).equals(elements.peek())){
				elements.pop();
			// 当前右括号和栈顶元素不匹配，或栈为空且还有右括号
			}else{
				return false;
			}
		}
		
		// 查看栈是否为空，栈里面不可能有右括号，主要查看是否还有左括号
		if(elements.empty()){
			return true;
		}else{
			return false;
		}
	}
}

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