Several bug fixes and improvements. (#887)

* fix the bugs of C code.

* Add a header figure.

* Improve the definition of tree node height.

codes/c/chapter_graph/graph_adjacency_list.c

@@ -224,9 +224,9 @@ void removeVertex(GraphAdjList *graph, unsigned int index) {
     // 将顶点前移
     for (int i = index; i < graph->size - 1; i++) {
         graph->vertices[i] = graph->vertices[i + 1]; // 顶点前移
-        graph->vertices[i]->pos--;                   // 所有前移的顶点索引值减1
+        graph->vertices[i]->pos--;                   // 所有前移的顶点索引值减 1
     }
-    graph->vertices[graph->size - 1] = 0; // 将被删除顶点的位置置 0
+    graph->vertices[graph->size - 1] = 0;
     graph->size--;
     // 释放内存
     freeVertex(vet);