【Leetcode-20】Valid Parentheses
/*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*/
public class Solution20 {
public boolean isValid(String s) {
Stack<Character> stack=new Stack<Character>();
int len=s.length();
for(int i=0;i<len;i++) {
if(s.charAt(i)=='['||s.charAt(i)=='('||s.charAt(i)=='{') {
stack.push(s.charAt(i));
}else if (s.charAt(i)==']') {
//从栈中取出一个元素
if(stack.empty()==true){
return false;
}
char ch=stack.pop();
if(ch!='[') {
return false;
}
}else if(s.charAt(i)==')') {
if(stack.empty()==true){
return false;
}
char ch=stack.pop();
if(ch!='(') {
return false;
}
}else if(s.charAt(i)=='}'){
if(stack.empty()==true){
return false;
}
char ch=stack.pop();
if(ch!='{') {
return false;
}
}
}
if(stack.empty()==true) {
return true;
}
return false;
}
}
