leetcode 20 :https://leetcode.com/problems/valid-parentheses/description/
如果 stack 为空时,pop()会 抛出exception, 处理的方法如下:
try { char top = st.pop(); if (! ( (top == '(' && s_i == ')') || (top == '{' && s_i == '}') || (top == '[' && s_i == ']') ) ) return false; } catch (Exception e) { return false; } }