Add binary heap (min-heap) implementation (#296)

Author: sora16bit

data_structures/binary_heap.r

@@ -0,0 +1,113 @@
+# A binary heap is a complete binary tree where each node is smaller than or
+# equal to its children (min-heap). The minimum is always at the root.
+#
+# To insert a value, add it as the last node and swap it upward
+# until the heap property holds (sift up).
+#
+# To remove the minimum, move the last node to the root and swap it
+# downward until the heap property holds (sift down).
+#
+# Time Complexities:
+# - Push (insert):  O(log n)
+# - Pop (extract min): O(log n)
+# - Peek: O(1)
+#
+# Space Complexity: O(n)
+#
+# Applications:
+# - Priority queues (task scheduling, process management)
+# - Dijkstra's shortest path algorithm
+# - Heap sort
+# - Finding the k smallest/largest elements (Top-K)
+# - Event-driven simulation
+#
+# Example usage:
+# h <- c()                 # create an empty heap
+# h <- push(h, 5)          # insert values
+# h <- push(h, 3)
+# h <- push(h, 8)
+# peek(h)                  # view the minimum (3) without removing
+# res <- pop(h)            # extract the minimum
+# res$value                # -> 3 (the minimum that was removed)
+# res$heap                 # -> the heap after removal
+
+
+push <- function(heap, value) {
+  
+  heap <- c(heap, value)
+  i <- length(heap)
+  
+  while (i > 1) {
+    
+    p <- i %/% 2
+    
+    if (heap[p] > heap[i]){
+      heap[c(p, i)] <- heap[c(i, p)]
+      i <- p
+    }
+    else {
+      break
+    }
+  }
+  return(heap)
+}
+
+pop <- function(heap) {
+  if (length(heap) == 0){
+    stop("Heap underflow: cannot pop from an empty heap")
+  }
+  
+  min_value <- heap[1]
+
+  heap[1] <- heap[length(heap)]
+  heap <- heap[-length(heap)]
+  
+  i <- 1
+  child <- 0
+  
+  while (2 * i <= length(heap)){
+    if ((2 * i + 1 <= length(heap) && heap[2 * i + 1] < heap[2 * i])){
+      child <- 2 * i + 1
+    }
+    else {
+      child <- 2 * i
+    }
+    
+    if (heap[child] < heap[i]){
+      heap[c(child, i)] <- heap[c(i, child)]
+      i <- child
+    }
+    else {
+      break
+    }
+  }
+  
+  return(list(value = min_value, heap = heap))
+}
+
+peek <- function(heap) {
+  if (length(heap) == 0){
+    stop("Heap underflow: cannot peek")
+  }
+  return(heap[1])
+}
+
+# Test
+if (sys.nframe() == 0){
+  h <- c()
+  h <- push(h, 5)
+  h <- push(h, 3)
+  h <- push(h, 8)
+  h <- push(h, 1)
+  h <- push(h, 9)
+  h <- push(h, 2)
+
+  print(peek(h))    
+  print(h)
+
+  res <- pop(h)
+  print(res$value)  
+  print(res$heap) 
+}
+  
+