20. Valid Parentheses

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Total Accepted: 95596
Total Submissions: 332615
Difficulty: Easy

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.

Java:

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public class Solution {
    public boolean isValid(String s) {
        int len = s.length();
        char[] stack = new char[len + 1];
        int top = 1;
        for (int i = 0; i < len; ++i) {
            char c = s.charAt(i);
            if (c == '(' || c == '[' || c == '{')
                stack[top++] = c;
            else if (c == ')' && stack[top - 1] != '(')
                return false;
            else if (c == ']' && stack[top - 1] != '[')
                return false;
            else if (c == '}' && stack[top - 1] != '{')
                return false;
            else
                --top;
        }
        if (top == 1)
            return true;
        return false;
    }
}

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