LeetCode 132 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.

分析：

判断括号合法性的问题，涉及栈的使用。

栈保存左括号，遇见左括号压栈，遇见右括号检查栈顶，匹配则弹栈，不匹配则说明不合法。

public class Solution {
    public boolean isValid(String s) {
        if(s==null || s.length()==0 )
            return false;
        Stack<Character> st = new Stack<Character>();
        for(int i=0; i<s.length(); i++){
            char ch = s.charAt(i);
            //左括号入栈
            if(ch=='(' || ch=='{' || ch=='[')
                st.push(ch);
            else{
                if(ch==')' && (st.empty()||st.peek()!='('))
                    return false;
                if(ch=='}' && (st.empty()||st.peek()!='{'))
                    return false;
                if(ch==']' && (st.empty()||st.peek()!='['))
                    return false;
                st.pop();
            }
        }
        //遍历完之后发现st非空
        if(!st.empty())
            return false;
        return true;
    }
}