Lintcode423-Valid Parentheses-Easy

423. 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.

Example

Example 1:

Input: "([)]"
Output: False

Example 2:

Input: "()[]{}"
Output: True

Challenge

Use O(n) time, n is the number of parentheses.

思路：

遍历String，左向括号入栈，当遇到右向括号时，（如果栈此时为空，则返回false）pop一个出来，并与右向括号匹配，看是否为一对。如果不是一对，则返回false。

遍历完整个数组，记得再检查栈是否为空，即左向括号数量=右向括号数量。所以  return (stack.isEmpty()); 

代码：

for (int i = 0; i < s.length(); i++) {
            if (s.charAt(i) == '(' || s.charAt(i) == '['||s.charAt(i) == '{')
                stack.push(s.charAt(i));
            else{
                if (stack.isEmpty()) 
                    return false;
                char pop = stack.pop();
                if (s.charAt(i) == ')' && (pop == '[' || pop == '{'))
                    return false;
                if (s.charAt(i) == ']' && (pop == '(' || pop == '{'))
                    return false;
                if (s.charAt(i) == '}' && (pop == '(' || pop == '['))
                    return false;
            }    
        }return stack.isEmpty();
    }