Question:
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.
Analysis:
问题描述：给出一个字符串，包含‘(‘, ‘)‘, ‘{‘, ‘}‘, ‘[‘ and ‘]‘确定它是否是有效地匹配括号。
思路：一看到括号匹配问题肯定想到用栈。遇到左括号就进栈；遇到右括号若栈顶元素与之匹配则POP出栈顶元素，若不匹配则返回false。
注意特殊情况：如"{}[]()", 或者“{[}]”, 或者“{{}}{{”等情况。
Answer：
public class Solution { public boolean isValid(String s) { char[] ch = s.toCharArray(); int n = ch.length; if(ch.length == 0 || ch.length % 2 != 0) return false; if(ch[0] == ‘}‘ || ch[0] == ‘]‘ || ch[0] == ‘)‘) return false; Stack<Character> st = new Stack<Character>(); st.add(ch[0]); int i = 1; while(!st.isEmpty() && i < n) { if(ch[i] == ‘{‘ || ch[i] == ‘[‘ || ch[i] == ‘(‘) { st.add(ch[i]); i++; } else { if(st.isEmpty()) return false; char c = st.pop(); if(c == ‘{‘ && ch[i] != ‘}‘ || c == ‘[‘ && ch[i] != ‘]‘ || c == ‘(‘ && ch[i] != ‘)‘) return false; i++; if(i < n && (ch[i] == ‘{‘ || ch[i] == ‘[‘ || ch[i] == ‘(‘)) { st.add(ch[i]); i++; } } } System.out.println(i +" " +st.size()); if(!st.isEmpty() || i < n - 1) return false; return true; } }
原文：http://www.cnblogs.com/little-YTMM/p/4872737.html