Binary tree postorder traversal

Signed-off-by: begeekmyfriend <begeekmyfriend@gmail.com>

Author: begeekmyfriend

145_binary_tree_postorder_traversal/Makefile

@@ -0,0 +1,2 @@
+all:
+	gcc -O2 -o test bst_postorder.c

145_binary_tree_postorder_traversal/bst_postorder.c

@@ -0,0 +1,76 @@
+#include <stdio.h>
+#include <stdlib.h>
+
+struct TreeNode {
+    int val;
+    struct TreeNode *left;
+    struct TreeNode *right;
+};
+
+struct node_backlog {
+    struct TreeNode *parent;
+    struct TreeNode *right;
+};
+
+/**
+ ** Return an array of size *returnSize.
+ ** Note: The returned array must be malloced, assume caller calls free().
+ **/
+static int* postorderTraversal(struct TreeNode* root, int* returnSize)
+{
+    if (root == NULL) {
+        return NULL;
+    }
+
+    int cap = 10000, count = 0;
+    int *results = malloc(cap * sizeof(int));
+    struct node_backlog *stack = malloc(cap / 16 * sizeof(*stack));
+    struct node_backlog *top = stack;
+    struct TreeNode *node = root;
+    struct node_backlog nbl;
+
+    while (node != NULL || top != stack) {
+        if (node == NULL) {
+            nbl = *--top;
+            if (nbl.right != NULL) {
+                node = nbl.right;
+                nbl.right = NULL;
+                *top++ = nbl;
+            } else {
+                node = nbl.parent;
+                results[count++] = node->val;
+                node = NULL;
+                continue;
+            }
+        }
+        nbl.parent = node;
+        nbl.right = node->right;
+        *top++ = nbl;
+        node = node->left;
+    }
+
+    *returnSize = count;
+    return results;
+}
+
+int main(int argc, char **argv)
+{
+    struct TreeNode root, node1, node2;
+    root.val = 1;
+    node1.val = 2;
+    node2.val = 3;
+    root.left = NULL;
+    root.right = &node1;
+    node1.left = &node2;
+    node1.right = NULL;
+    node2.left = NULL;
+    node2.right = NULL;
+
+    int i, count = 0;
+    int *results = postorderTraversal(&root, &count);
+    for (i = 0; i < count; i++) {
+        printf("%d ", results[i]);
+    }
+    printf("\n");
+    return 0;
+}