Title description
Given a string containing only the characters'(',')','{','}','[' and']', determine whether the given string is a legal bracket sequence. The
brackets must be correct The order is closed, "()" and "()[]{}" are both legal bracket sequences, but "(]" and "([)]" are illegal.
Example 1
enter
"["
return value
false
Example 2
enter
"[]"
return value
true
import java.util.*;
public class Solution {
/**
* 有效的括号
* <p>
* 给定一个只包括 '(',')','{','}','[',']' 的字符串,判断字符串是否有效。
* 有效字符串需满足:
* 左括号必须用相同类型的右括号闭合。
* 左括号必须以正确的顺序闭合。
* 注意空字符串可被认为是有效字符串。
* 示例 1:
* 输入: "()"
* 输出: true
* 示例 2:
* 输入: "()[]{}"
* 输出: true
* 示例 3:
* 输入: "(]"
* 输出: false
* 示例 4:
* 输入: "([)]"
* 输出: false
* 示例 5:
* 输入: "{[]}"
* 输出: true
*/
public boolean isValid (String s) {
if(s==null || s.length()==0){
return true;
}
Stack<Character> stack = new Stack<Character>();
char[] chars = s.toCharArray();
for(int i=0;i<chars.length;i++){
if('[' == chars[i]){
stack.push(']');
}else if('{' == chars[i]){
stack.push('}');
}else if('(' == chars[i]){
stack.push(')');
}else if(stack.isEmpty() || stack.pop()!=chars[i]){//if else if只能走一个分支
return false;
}
}
//都弹出完了 才算全部匹配
return stack.isEmpty();
}
}