<p>There are a total of <code>numCourses</code> courses you have to take, labeled from <code>0</code> to <code>numCourses - 1</code>. You are given an array <code>prerequisites</code> where <code>prerequisites[i] = [a<sub>i</sub>, b<sub>i</sub>]</code> indicates that you <strong>must</strong> take course <code>b<sub>i</sub></code> first if you want to take course <code>a<sub>i</sub></code>.</p>

<ul>
	<li>For example, the pair <code>[0, 1]</code>, indicates that to take course <code>0</code> you have to first take course <code>1</code>.</li>
</ul>

<p>Return <code>true</code> if you can finish all courses. Otherwise, return <code>false</code>.</p>

<p>&nbsp;</p>
<p><strong class="example">Example 1:</strong></p>

<pre>
<strong>Input:</strong> numCourses = 2, prerequisites = [[1,0]]
<strong>Output:</strong> true
<strong>Explanation:</strong> There are a total of 2 courses to take. 
To take course 1 you should have finished course 0. So it is possible.
</pre>

<p><strong class="example">Example 2:</strong></p>

<pre>
<strong>Input:</strong> numCourses = 2, prerequisites = [[1,0],[0,1]]
<strong>Output:</strong> false
<strong>Explanation:</strong> There are a total of 2 courses to take. 
To take course 1 you should have finished course 0, and to take course 0 you should also have finished course 1. So it is impossible.
</pre>

<p>&nbsp;</p>
<p><strong>Constraints:</strong></p>

<ul>
	<li><code>1 &lt;= numCourses &lt;= 2000</code></li>
	<li><code>0 &lt;= prerequisites.length &lt;= 5000</code></li>
	<li><code>prerequisites[i].length == 2</code></li>
	<li><code>0 &lt;= a<sub>i</sub>, b<sub>i</sub> &lt; numCourses</code></li>
	<li>All the pairs prerequisites[i] are <strong>unique</strong>.</li>
</ul>

class Solution:
    def chkValue(self, a, b):

        if b not in self.kv:
            return True
        else:
            nv = self.kv[b]
            if a in nv:
                return False
            else:
                for c in nv:
                    if not self.chkValue(a, c):
                        return False 
                return True

    def delcourse(self, i):
        s = 0
        #if i == 0:
        #    print('p', self.p)
        #    print('q', self.q)
        while True:
            try:
                idx = self.q.index(i)
                #if i == 0:
                #    print('idx', idx)
                self.q.pop(idx)
                c = self.p.pop(idx)
                # if i == 0:
                #    print('idx', idx)
                if c not in self.p:
                    self.delcourse(c)
            except:
                break
        
        #if i == 0:
        #    print('__p', self.p)
        #    print('__q', self.q)

    def canFinish(self, numCourses: int, prerequisites: List[List[int]]) -> bool:
        self.p = [c[0] for c in prerequisites]
        self.q = [c[1] for c in prerequisites]
        ilen = len(self.q)
        # self.ps = set(p)
        for i in range(numCourses):
            if i not in self.p:                
                self.delcourse(i)
        #print('p', self.p)
        #print('q', self.q)

        return self.p == []



        
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