Add Cycle Detection (#22)

Author: frankymacster

Simple Recursive/Cycle Detection/README.md

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+# Cycle Detection
+"Floyd's cycle-finding algorithm is a pointer algorithm that uses only two pointers, which move through the sequence at different speeds"
+
+## References
+* [Wikipedia](https://en.wikipedia.org/wiki/Cycle_detection#Floyd's_Tortoise_and_Hare)

Simple Recursive/Cycle Detection/code.js

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+// import visualization libraries {
+const { Array2DTracer, Layout, LogTracer, GraphTracer, Tracer, VerticalLayout } = require('algorithm-visualizer');
+// }
+
+
+// define tracer variables {
+function ListNode(val) {
+    this.val = val
+    this.next = null
+}
+
+const node0 = new ListNode(0)
+const node1 = new ListNode(1)
+const node2 = new ListNode(2)
+const node3 = new ListNode(3)
+const node4 = new ListNode(4)
+const node5 = new ListNode(5)
+const node6 = new ListNode(6)
+
+const list = node0
+list.next = node1
+list.next.next = node2
+list.next.next.next = node3
+list.next.next.next.next = node4
+list.next.next.next.next.next = node5
+list.next.next.next.next.next.next = node6
+list.next.next.next.next.next.next.next = node2
+
+const graphTracer = new GraphTracer("Linked List").directed()
+const logTracer = new LogTracer("Console")
+Layout.setRoot(new VerticalLayout([graphTracer, logTracer]))
+
+graphTracer.addNode(node0.val)
+graphTracer.addNode(node1.val)
+graphTracer.addNode(node2.val)
+graphTracer.addNode(node3.val)
+graphTracer.addNode(node4.val)
+graphTracer.addNode(node5.val)
+graphTracer.addNode(node6.val)
+graphTracer.addEdge(node0.val, node1.val)
+graphTracer.addEdge(node1.val, node2.val)
+graphTracer.addEdge(node2.val, node3.val)
+graphTracer.addEdge(node3.val, node4.val)
+graphTracer.addEdge(node4.val, node5.val)
+graphTracer.addEdge(node5.val, node6.val)
+graphTracer.addEdge(node6.val, node2.val)
+Tracer.delay()
+// }
+
+var listHasCycle = function(head) {
+    // visualize {
+    graphTracer.select(head.val)
+    graphTracer.visit(head.val)
+    Tracer.delay()
+    graphTracer.deselect(head.val)
+    graphTracer.leave(head.val)
+    // }
+    
+    // 1. is there a cycle?
+    let slow = head.next
+    let fast = head.next.next
+    // visualize {
+    graphTracer.select(slow.val)
+    graphTracer.visit(fast.val)
+    Tracer.delay()
+    graphTracer.deselect(slow.val)
+    graphTracer.leave(fast.val)
+    // }
+    while (slow !== fast) {
+        slow = slow.next
+        fast = fast.next.next
+        // visualize {
+        graphTracer.select(slow.val)
+        graphTracer.visit(fast.val)
+        Tracer.delay()
+        graphTracer.deselect(slow.val)
+        graphTracer.leave(fast.val)
+        // }
+    }
+
+    // 2. where does the cycle start?
+    let cycleStartPosition = 0
+    slow = head
+    // visualize {
+    graphTracer.select(slow.val)
+    graphTracer.visit(fast.val)
+    Tracer.delay()
+    graphTracer.deselect(slow.val)
+    graphTracer.leave(fast.val)
+    // }
+    while (slow !== fast) {
+        slow = slow.next
+        fast = fast.next
+        cycleStartPosition += 1
+        // visualize {
+        graphTracer.select(slow.val)
+        graphTracer.visit(fast.val)
+        Tracer.delay()
+        graphTracer.deselect(slow.val)
+        graphTracer.leave(fast.val)
+        // }
+    }
+
+    // 3. what is the length of the cycle?
+    let cycleLength = 1
+    fast = slow.next
+    // visualize {
+    graphTracer.select(slow.val)
+    graphTracer.visit(fast.val)
+    Tracer.delay()
+    graphTracer.deselect(slow.val)
+    graphTracer.leave(fast.val)
+    // }
+    while (slow !== fast) {
+        fast = fast.next
+        cycleLength += 1
+        // visualize {
+        graphTracer.select(slow.val)
+        graphTracer.visit(fast.val)
+        Tracer.delay()
+        graphTracer.deselect(slow.val)
+        graphTracer.leave(fast.val)
+        // }
+    }
+
+    return {
+      cycleLength,
+      cycleStartPosition,
+    }
+}
+// log {
+const res = listHasCycle(list)
+logTracer.print(`cycle start position: ${res.cycleStartPosition}`)
+logTracer.print("\n")
+logTracer.print(`cycle length: ${res.cycleLength}`)
+// }