[LeetCode]Longest Valid Parentheses

原创于 2014-09-13 20:01:24 发布 · 631 阅读

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本文介绍了一种使用栈来解决寻找字符串中最长有效括号子串问题的方法。通过跟踪未匹配右括号的位置和已匹配括号的索引，算法能够高效地找到最长的有效括号子串长度。

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Given a string containing just the characters '(' and ')', find the length of the longest valid (well-formed) parentheses substring.

For "(()", the longest valid parentheses substring is "()", which has length = 2.

Another example is ")()())", where the longest valid parentheses substring is "()()", which has length = 4.

public class Solution {
    public int longestValidParentheses(String s) {
    	if(s==null||s.length()==0){
    		return 0;
    	}
    	Stack<Integer> stack = new Stack<Integer>();
    	int lastposition = -1;// the last  not match position of ")"
    	int res = 0;
    	for(int i =0;i<s.length();i++){
    		String str = s.substring(i,i+1);
    		if(str.equals("(")){
    			stack.push(i);
    		}else{
    			if(stack.isEmpty()){
    				lastposition = i;
    			}else{
    				stack.pop();
    				if(stack.isEmpty()){
    					res = Math.max(res, i-lastposition);
    				}else{
    					res = Math.max(res, i-stack.peek());//s.peek()== last not match postion of "("
    				}
    			} 
    		}
    	}
        return res;
    }
}