我正在尝试实现红黑树的CLRS伪代码。当我尝试运行该程序时,会抛出NullPointerException
。请查看代码并找出其中的错误。任何进一步的建议都是受欢迎的。
public class RedBlackTree {
Node nil;
Node root;
String RED = "red";
String BLACK = "black";
public void left_rotate(RedBlackTree T, Node x) {
Node y = x.right;
x.right = y.left;
if (y.left != T.nil)
y.left.parent = x;
y.parent = x.parent;
if (x.parent == T.nil)
T.root = y;
else if (x == x.parent.left)
x.parent.left = y;
else
x.parent.right = y;
y.left = x;
x.parent = y;
}
public void right_rotate(RedBlackTree T, Node x) {
Node y = x.left;
x.left = y.right;
if (y.right != T.nil)
y.right.parent = x;
y.parent = x.parent;
if (x.parent == T.nil)
T.root = y;
else if (x == x.parent.right)
x.parent.right = y;
else
x.parent.left = y;
y.right = x;
x.parent = y;
}
public void rb_insert_fixup(RedBlackTree T, Node z) {
while (z.parent.color == RED) {
if (z.parent == z.parent.parent.left) {
Node y = z.parent.parent.right;
if (y.color == RED) {
z.parent.color = BLACK;
y.color = BLACK;
z.parent.parent.color = RED;
z = z.parent.parent;
} else {
if (z == z.parent.right) {
z = z.parent;
left_rotate(T, z);
}
z.parent.color = BLACK;
z.parent.parent.color = RED;
right_rotate(T, z.parent.parent);
}
} else {
Node y = z.parent.parent.left;
if (y.color == RED) {
z.parent.color = BLACK;
y.color = BLACK;
z.parent.parent.color = RED;
z = z.parent.parent;
} else {
if (z == z.parent.left) {
z = z.parent;
right_rotate(T, z);
}
z.parent.color = BLACK;
z.parent.parent.color = RED;
left_rotate(T, z.parent.parent);
}
}
}
T.root.color = BLACK;
}
public void insert(RedBlackTree T, Node z) {
Node y = T.nil;
Node x = T.root;
while (x != T.nil) {
y = x;
if (z.key < x.key)
x = x.left;
else
x = x.right;
}
z.parent = y;
if (y == T.nil)
T.root = z;
else if (z.key < y.key)
y.left = z;
else
y.right = z;
z.left = T.nil;
z.right = T.nil;
z.color = RED;
rb_insert_fixup(T, z);
}
public void inorder_tree_walk(Node x) {
if (x != null) {
inorder_tree_walk(x.left);
System.out.println(x.key + ":" + x.color + " ");
inorder_tree_walk(x.right);
}
}
public static void main(String[] args) {
RedBlackTree rbt = new RedBlackTree();
Node a = new Node(12, "a");
rbt.insert(rbt, a);
Node b = new Node(5, "b");
rbt.insert(rbt, b);
Node c = new Node(18, "c");
rbt.insert(rbt, c);
Node d = new Node(2, "d");
rbt.insert(rbt, d);
Node e = new Node(9, "e");
rbt.insert(rbt, e);
rbt.inorder_tree_walk(rbt.root);
}
}
class Node {
int key;
String data;
String color;
Node left, right, parent;
public Node(int key, String data) {
this.key = key;
this.data = data;
}
}
StackTrace是:
线程“main”java.lang.NullPointerException中的异常 在RedBlackTree.rb_insert_fixup(RedBlackTree.java:42) 在RedBlackTree.insert(RedBlackTree.java:102) 在RedBlackTree.main(RedBlackTree.java:117)
答案 0 :(得分:0)
您必须初始化nil
和root
:
public class RedBlackTree {
Node nil; <-- is null
Node root; <-- is null
否则,这也是null:
while (z.parent.color == RED) { <-- z.parent is null