Why not index instead of q.top()? [leetcode 32][dynamic programming]

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class Solution {
public:
  int longestValidParentheses(string s) {
    stack<int> q; //LIFO
    int start = 0;
    int ans = 0;
    for (int i = 0;i < s.length(); i++) {
      if(s[i] == '(') {
        q.push(i);
      } else {
        if (q.empty()) {
          start = i + 1;
        } else {
          int index = q.top(); q.pop();
          ans = max(ans, q.empty() ? i - start + 1 : i - q.top());

if don't need

index

, why declare it?

        }
      }
    }
    return ans;
  }
};

What I have tried:

(why not choose the matched ‘(‘ but the previous one?)

Posted 24-Sep-20 20:31pm

JacksonSteel

Updated 24-Sep-20 23:45pm

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OriginalGriff · Answer 1 · 2020-09-24T20:53:00

You'd have to ask the original author, but it's either "I was going to do something with it, but didn't need to", "I'll remove it", or "It was useful for debugging".

The latter is the most likely: it makes it a lot easier to see what was the top item before it is removed when testing your code in the debugger.
I do it often: break down a complex line of code into lots of simpler ones each saving an intermediate result in a variable so they are easier to see. When it's all working, put it all back together and test it as a single item.

Sometimes you forget to remove the intermediate variables, is all.

Patrice T · Answer 2 · 2020-09-24T23:00:00

Solution 2

Quote:
(why not choose the matched ‘(‘ but the previous one?)

We can't help you because the code you provided is not a complete function.
As OG said only the author of that code can explain you what is what.

Quote:
why declare it?

If you can compile the code after commenting the line, then you don't need it.

Posted 24-Sep-20 23:00pm

Patrice T

Updated 24-Sep-20 23:45pm

v2

Comments

KarstenK 25-Sep-20 5:42am

Bold words, if he comments out the line a q.pop() will be missing and the code wont run correctly. :-O

KarstenK · Answer 3 · 2020-09-24T23:45:00

Solution 3

Best is to understand the code. A developer should understand every single line of its code.

You dont need that index so you may remove that command, BUT the code has the flaw that a q.pop() is on that line.

I guess that the function will need it :-O

Posted 24-Sep-20 23:45pm

KarstenK