Valid Parentheses

Given a string containing just the characters`'('`,`')'`,`'{'`,`'}'`,`'['`and`']'`, determine if the input string is valid.

An input string is valid if:

1. Open brackets must be closed by the same type of brackets.
2. 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
``````

Solution

``````class Solution {
public boolean isValid(String s) {
if (s == null) {
return false;
}
Stack < Character > stack = new Stack < Character > ();
for (int i = 0; i < s.length(); i++) {
char ch = s.charAt(i);
if (!stack.isEmpty() && match(stack.peek(), ch)) {
stack.pop();
} else {
stack.push(ch);
}
}
return stack.isEmpty();
}
boolean match(char ch1, char ch2) {
return (ch1 == '(' && ch2 == ')') ||
(ch1 == '[' && ch2 == ']') ||
(ch1 == '{' && ch2 == '}');
}
}
``````
``````class Solution {
public boolean isValid(String s) {
Stack<Character> stack = new Stack<>();
for(char c : s.toCharArray()){
switch(c){
case ']': if(stack.isEmpty() || stack.pop()!='[') return false; break;
case ')': if(stack.isEmpty() || stack.pop()!='(') return false; break;
case '}': if(stack.isEmpty() || stack.pop()!='{') return false; break;
default: stack.push(c);
}
}
return stack.isEmpty();
}
}
``````