Lintcode423-Valid Parentheses-Easy

思路：

数据结构：stack。遍历整个字符串，如果遇到左向括号( [ { 则入栈。如果遇到右向括号时，先检查栈是否为空，为空说明左右向括号数目不一致，返回false；不为空则弹出栈顶元素查看是否和右向括号匹配。遍历完，要return stack.isEmpty(); 确保栈内没有剩余。

代码：

public class Solution {
    /**
     * @param s: A string
     * @return: whether the string is a valid parentheses
     */
    public boolean isValidParentheses(String s) {
        Stack<Character> st = new Stack<Character>();
        
        for (int i = 0; i < s.length(); i++) {
            char c = s.charAt(i);
            if (c == '(' || c == '[' || c == '{') {
                st.push(c);
            }
            if (c == ')' || c == ']' || c == '}') {
                if (st.isEmpty()) {
                    return false;
                }
                char popChar = st.pop();
                if ( c == ')' && popChar != ')') {
                    return false;
                } 
                if ( c == ']' && popChar != ']') {
                    return false;
                }
                if ( c == '}' && popChar != '}' ) {
                    return false;
                }
            }
        }
    return st.isEmpty();
    }
}

代码风格优化：String遍历每一个char（line 9) ; 用if-else分支更准确（line 10, 12)

public class Solution {
    /**
     * @param s: A string
     * @return: whether the string is a valid parentheses
     */
    public boolean isValidParentheses(String s) {
        Stack<Character> st = new Stack<Character>();
        
        for (Character c : s.toCharArray()) {
            if (c == '(' || c == '[' || c == '{') {
                st.push(c);
            } else {
                if (st.isEmpty()) {
                    return false;
                }
                char popChar = st.pop();
                if ( c == ')' && popChar != '(') {
                    return false;
                } 
                if ( c == ']' && popChar != '[') {
                    return false;
                }
                if ( c == '}' && popChar != '{') {
                    return false;
                }
            }
        }
    return st.isEmpty();
    }
}