20. Valid Parentheses

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本文介绍了一种使用栈数据结构来验证括号是否正确配对的方法。该方法适用于包含圆括号'()',方括号'[]'及花括号'{}

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.

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public class Solution{
	public boolean isValid(String s) {
		if (s == null || s.length() == 0) {
			return true;		//先简单判断
		}
		char[] stack = new char[s.length()];//用来放[({
		int top = 0;//表示当前符号的下标
		for (int i=0, j=s.length(); i<j; i++) {
			char c = s.charAt(i);
			if (c == '[' || c == '(' || c == '{') {
				stack[top++] = c;
			} else if (c == ']' || c == ')' || c == '}') {
				if(top == 0) {	//stack为空
					return false;
				}
				char pc = stack[--top];
				if ((pc == '[' && c == ']') || 
				    (pc == '(' && c == ')') || 
				    (pc == '{' && c == '}'))
					continue;	//符号配对成功，继续执行程序
				} else {
					return false;
				}
			} else {
				return false;
			}
		}
		return top == 0;//如果当前符号的下标是在0，则表示成功，否则必定失败
	}
}

其实可以思考着把配对好的符号移除~