# 20. Valid Parentheses

[Link](https://leetcode.com/problems/valid-parentheses/description/)

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**

用stack解

```
    public boolean isValid(String s) {
        Stack<Character> stack = new Stack();
        for(char c : s.toCharArray()){
            if(c == '(' ||c == '[' ||c == '{'){
                stack.push(c);
            }
            else{
                if(stack.isEmpty()) return false;
                if (c == ')'){
                    if(stack.isEmpty()) return false;
                    char temp = stack.pop();
                    if(temp != '(') return false;
                }
                else if (c == ']'){
                    if(stack.isEmpty()) return false;
                    char temp = stack.pop();
                    if(temp != '[') return false;
                }
                else if (c == '}'){
                    if(stack.isEmpty()) return false;
                    char temp = stack.pop();
                    if(temp != '{') return false;
                }
                else{
                    return false;
                }
            } 
        }
        return stack.isEmpty();
    }
```
