feat: add LeetCode problem 100

Check for tree equivalence

Author: Rich Vigorito

leetcode/src/100.c

@@ -0,0 +1,17 @@
+/**
+ * 100. Same Tree
+ * https://leetcode.com/problems/same-tree/
+ *
+ * Given the roots of two binary trees p and q, write a function to check if they are the same or not.
+ * Two binary trees are considered the same if they are structurally identical, and the nodes have the same value.
+ *
+ */
+
+bool isSameTree(struct TreeNode* p, struct TreeNode* q) {
+    if (p == NULL && q == NULL) return true;
+    if (p == NULL || q == NULL) return false;
+    if (p->val != q->val)       return false;
+
+    return isSameTree(p->left, q->left) 
+        && isSameTree(p->right, q->right);
+}