From c168638060c2755ef5e5450b8e827a4f44afa41b Mon Sep 17 00:00:00 2001
From: Leo Yang <junjiey3@illinois.edu>
Date: Fri, 24 Jan 2020 23:14:25 -0600
Subject: [PATCH] feat: add union find algorithm (aka disjoint set)

---
 data_structure/disjoint_set.cpp | 69 +++++++++++++++++++++++++++++++++
 1 file changed, 69 insertions(+)
 create mode 100644 data_structure/disjoint_set.cpp

diff --git a/data_structure/disjoint_set.cpp b/data_structure/disjoint_set.cpp
new file mode 100644
index 000000000..de5d1c0e0
--- /dev/null
+++ b/data_structure/disjoint_set.cpp
@@ -0,0 +1,69 @@
+#include <iostream>
+#include <vector>
+using namespace std;
+
+vector<int> root, rnk;
+
+void CreateSet(int n){
+    root = vector<int> (n+1);
+    rnk = vector<int> (n+1, 1);
+    
+    for(int i = 1; i <= n; ++ i) {
+        root[i] = i;
+    }
+}
+
+int Find(int x) {
+    if (root[x] == x) {
+        return x;
+    }
+    return root[x] = Find(root[x]);
+}
+
+bool InSameUnion(int x, int y) {
+    return Find(x) == Find(y);
+}
+
+void Union(int x, int y) {
+    int a = Find(x), b = Find(y);
+    if(a != b) {
+        if (rnk[a] < rnk[b]) {
+            root[a] = b;
+        } else if (rnk[a] > rnk[b]) {
+            root[b] = a;
+        } else {
+            root[a] = b;
+            ++ rnk[b];
+        }
+    }
+}
+
+int main() {
+    
+    // tests CreateSet & Find
+    
+    int n = 100;
+    CreateSet(n);
+    
+    for (int i = 1; i <= 100; ++ i) {
+        if (root[i] != i) {
+            cout << "Fail" << endl;
+            break;
+        }
+    }
+    
+    // tests InSameUnion & Union
+    
+    cout << "1 and 2 are initially not in the same subset" << endl;
+    if (InSameUnion(1, 2)) {
+        cout << "Fail" << endl;
+    }
+        
+    Union(1, 2);
+    cout << "1 and 2 are now in the same subset" << endl;
+    if (!InSameUnion(1, 2)) {
+        cout << "Fail" << endl;
+    }
+    
+    return 0;
+}