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-rw-r--r--count_unreach_node.c53
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diff --git a/count_unreach_node.c b/count_unreach_node.c
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+// 2316. Count Unreachable Pairs of Nodes in an Undirected Graph
+#include <stdio.h>
+#include <stdlib.h>
+
+/*
+ * given integer 'n', there is undirected graph with 'n' nodes, numbered 0 to 'n
+ * - 1' given 2d integer array 'edges' where 'edges[i] = [ai bi]' denotes that
+ * there exists an undirected edge connecting nodes ai and bi. return number of
+ * pairs of different nodes that are unreachable from each other
+ */
+
+int union_find(int *u, int val) {
+ if (u[val] == val)
+ return val;
+ else
+ return union_find(u, u[val]);
+}
+
+void union_set(int *u, int *u_size, int a, int b) {
+ a = union_find(u, a);
+ b = union_find(u, b);
+ if (a == b)
+ return;
+ if (u_size[a] > u_size[b]) {
+ u[b] = u[a];
+ u_size[a] += u_size[b];
+ } else {
+ u[a] = u[b];
+ u_size[b] += u_size[a];
+ }
+}
+
+long long countPairs(int n, int **edges, int edges_size, int *edges_col_size) {
+ int *u = malloc(n * sizeof(int)), *u_size = malloc(n * sizeof(int));
+ for (int i = 0; i < n; i++) {
+ u[i] = i;
+ u_size[i] = 1;
+ }
+ for (int i = 0; i < edges_size; i++)
+ union_set(u, u_size, edges[i][0], edges[i][1]);
+ long long ans = 0;
+ for (int i = 0; i < n; i++)
+ ans += n - u_size[union_find(u, i)];
+ return ans / 2;
+}
+
+int main() {
+ int n1 = 3, e1[][3] = {{0, 1}, {0, 2}, {1, 2}}, es1 = 3, ecs1[] = {3};
+ int n2 = 7, e2[][5] = {{0, 2}, {0, 5}, {2, 4}, {1, 6}, {5, 4}}, es2 = 5,
+ ecs2[] = {5};
+ printf("%lld\n", countPairs(n1, e1, es1, ecs1)); // expect: 0
+ printf("%lld\n", countPairs(n2, e2, es2, ecs2)); // expect: 0
+}