### Problem

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

#### Example

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.

### JavaScript Code

function longestValidParentheses(s) { var stack = []; var result = 0; for(var i=0; i<=s.length-1; i++){ var c = s.charAt(i); if(c=='('){ var a = [i,0]; stack.push(a); }else{ if(stack.length == 0||stack[stack.length - 1][1]==1){ var a = [i,1]; stack.push(a); }else{ var temp = stack.pop(); var currentLen=0; if(stack.length == 0){ currentLen = i+1; }else{ currentLen = i-stack[stack.length - 1][0]; } result = Math.max(result, currentLen); } } } return result; }

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