LeetCode----20. Valid Parentheses

Given a string containing just the characters 

'('

, 

')'

, 

'{'

, 

'}'

, 

'['

 and 

']'

,
determine if the input string is valid.
The brackets must close in the correct order, 

"()"

 and 

"()[]{}"

 are
all valid but 

"(]"

 and 

"([)]"

 are
not.

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"()[]"  "("    "(()[])"   "([()])"

Runtime Error Message:Line
8: java.lang.StringIndexOutOfBoundsException: String index out of range: 2

Last executed input:"(("
"[({(())}[()])]"

public class Solution {
public boolean isValid(String s) {
Stack<Character> stack = new Stack<Character>();
for (Character c : s.toCharArray()) {
if ("({[".contains(String.valueOf(c))) {
stack.push(c);
} else {
if (!stack.isEmpty() && is_valid(stack.peek(), c)) {
stack.pop();
} else {
return false;
}
}
}
return stack.isEmpty();
}

private boolean is_valid(char c1, char c2) {
return (c1 == '(' && c2 == ')') || (c1 == '{' && c2 == '}')
|| (c1 == '[' && c2 == ']');
}
}