比如輸入圖 4.12 中左邊的二叉搜索樹(shù),則輸出轉(zhuǎn)換之后的排序現(xiàn)向鏈表。
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public static class BinaryTreeNode {
int value;
BinaryTreeNode left;
BinaryTreeNode right;
}
在二叉樹(shù)中,每個(gè)結(jié)點(diǎn)都有兩個(gè)指向子結(jié)點(diǎn)的指針。在雙向鏈表中,每個(gè)結(jié)點(diǎn)也有兩個(gè)指針,它們分別指向前一個(gè)結(jié)點(diǎn)和后一個(gè)結(jié)點(diǎn)。由于這兩種結(jié)點(diǎn)的結(jié)構(gòu)相似,同時(shí)二叉搜索樹(shù)也是一種排序的數(shù)據(jù)結(jié)構(gòu),因此在理論上有可能實(shí)現(xiàn)二叉搜索樹(shù)和排序的雙向鏈表的轉(zhuǎn)換。在搜索二叉樹(shù)中,左子結(jié)點(diǎn)的值總是小于父結(jié)點(diǎn)的值,右子結(jié)點(diǎn)的值總是大于父結(jié)點(diǎn)的值。因此我們?cè)谵D(zhuǎn)換成排序雙向鏈表時(shí),原先指向左子結(jié)點(diǎn)的指針調(diào)整為鏈表中指向前一個(gè)結(jié)點(diǎn)的指針,原先指向右子結(jié)點(diǎn)的指針調(diào)整為鏈表中指向后一個(gè)結(jié)點(diǎn)指針。接下來(lái)我們考慮該如何轉(zhuǎn)換。
由于要求轉(zhuǎn)換之后的鏈表是排好序的,我們可以中序遍歷樹(shù)中的每一個(gè)結(jié)點(diǎn), 這是因?yàn)橹行虮闅v算法的特點(diǎn)是按照從小到大的順序遍歷二叉樹(shù)的每一個(gè)結(jié)點(diǎn)。當(dāng)遍歷到根結(jié)點(diǎn)的時(shí)候,我們把樹(shù)看成三部分:值為 10 的結(jié)點(diǎn)、根結(jié)點(diǎn)值為 6 的左子樹(shù)、根結(jié)點(diǎn)值為 14 的右子樹(shù)。根據(jù)排序鏈表的定義,值為 10 的結(jié)點(diǎn)將和它的左子樹(shù)的最大一個(gè)結(jié)點(diǎn)(即值為 8 的結(jié)點(diǎn))鏈接起來(lái),同時(shí)它還將和右子樹(shù)最小的結(jié)點(diǎn)(即值為 12 的結(jié)點(diǎn))鏈接起來(lái),如圖 4.13 所示。
http://wiki.jikexueyuan.com/project/for-offer/images/39.png" alt="" />
按照中序遍歷的順序, 當(dāng)我們遍歷轉(zhuǎn)換到根結(jié)點(diǎn)(值為 10 的結(jié)點(diǎn))時(shí),它的左子樹(shù)已經(jīng)轉(zhuǎn)換成一個(gè)排序的鏈表了, 并且處在鏈表中的最后一個(gè)結(jié)點(diǎn)是當(dāng)前值最大的結(jié)點(diǎn)。我們把值為 8 的結(jié)點(diǎn)和根結(jié)點(diǎn)鏈接起來(lái),此時(shí)鏈表中的最后一個(gè)結(jié)點(diǎn)就是 10 了。接著我們?nèi)サ貧v轉(zhuǎn)換右子樹(shù), 并把根結(jié)點(diǎn)和右子樹(shù)中最小的結(jié)點(diǎn)鏈接起來(lái)。至于怎么去轉(zhuǎn)換它的左子樹(shù)和右子樹(shù),由于遍歷和轉(zhuǎn)換過(guò)程是一樣的,我們很自然地想到可以用遞歸。
public class Test27 {
/**
* 二叉樹(shù)的樹(shù)結(jié)點(diǎn)
*/
public static class BinaryTreeNode {
int value;
BinaryTreeNode left;
BinaryTreeNode right;
}
/**
* 題目:輸入一棵二叉搜索樹(shù),將該二叉搜索樹(shù)轉(zhuǎn)換成一個(gè)排序的雙向鏈表。
* 要求不能創(chuàng)建任何新的結(jié)點(diǎn),只能調(diào)整樹(shù)中結(jié)點(diǎn)指針的指向。
*
* @param root 二叉樹(shù)的根結(jié)點(diǎn)
* @return 雙向鏈表的頭結(jié)點(diǎn)
*/
public static BinaryTreeNode convert(BinaryTreeNode root) {
// 用于保存處理過(guò)程中的雙向鏈表的尾結(jié)點(diǎn)
BinaryTreeNode[] lastNode = new BinaryTreeNode[1];
convertNode(root, lastNode);
// 找到雙向鏈表的頭結(jié)點(diǎn)
BinaryTreeNode head = lastNode[0];
while (head != null && head.left != null) {
head = head.left;
}
return head;
}
/**
* 鏈表表轉(zhuǎn)換操作
*
* @param node 當(dāng)前的根結(jié)點(diǎn)
* @param lastNode 已經(jīng)處理好的雙向鏈表的尾結(jié)點(diǎn),使用一個(gè)長(zhǎng)度為1的數(shù)組,類似C++中的二級(jí)指針
*/
public static void convertNode(BinaryTreeNode node, BinaryTreeNode[] lastNode) {
// 結(jié)點(diǎn)不為空
if (node != null) {
// 如果有左子樹(shù)就先處理左子樹(shù)
if (node.left != null) {
convertNode(node.left, lastNode);
}
// 將當(dāng)前結(jié)點(diǎn)的前驅(qū)指向已經(jīng)處理好的雙向鏈表(由當(dāng)前結(jié)點(diǎn)的左子樹(shù)構(gòu)成)的尾結(jié)點(diǎn)
node.left = lastNode[0];
// 如果左子樹(shù)轉(zhuǎn)換成的雙向鏈表不為空,設(shè)置尾結(jié)點(diǎn)的后繼
if (lastNode[0] != null) {
lastNode[0].right = node;
}
// 記錄當(dāng)前結(jié)點(diǎn)為尾結(jié)點(diǎn)
lastNode[0] = node;
// 處理右子樹(shù)
if (node.right != null) {
convertNode(node.right, lastNode);
}
}
}
public static void main(String[] args) {
test01();
test02();
test03();
test04();
test05();
}
private static void printList(BinaryTreeNode head) {
while (head != null) {
System.out.print(head.value + "->");
head = head.right;
}
System.out.println("null");
}
private static void printTree(BinaryTreeNode root) {
if (root != null) {
printTree(root.left);
System.out.print(root.value + "->");
printTree(root.right);
}
}
// 10
// / \
// 6 14
// /\ /\
// 4 8 12 16
private static void test01() {
BinaryTreeNode node10 = new BinaryTreeNode();
node10.value = 10;
BinaryTreeNode node6 = new BinaryTreeNode();
node6.value = 6;
BinaryTreeNode node14 = new BinaryTreeNode();
node14.value = 14;
BinaryTreeNode node4 = new BinaryTreeNode();
node4.value = 4;
BinaryTreeNode node8 = new BinaryTreeNode();
node8.value = 8;
BinaryTreeNode node12 = new BinaryTreeNode();
node12.value = 12;
BinaryTreeNode node16 = new BinaryTreeNode();
node16.value = 16;
node10.left = node6;
node10.right = node14;
node6.left = node4;
node6.right = node8;
node14.left = node12;
node14.right = node16;
System.out.print("Before convert: ");
printTree(node10);
System.out.println("null");
BinaryTreeNode head = convert(node10);
System.out.print("After convert : ");
printList(head);
System.out.println();
}
// 5
// /
// 4
// /
// 3
// /
// 2
// /
// 1
private static void test02() {
BinaryTreeNode node1 = new BinaryTreeNode();
node1.value = 1;
BinaryTreeNode node2 = new BinaryTreeNode();
node2.value = 2;
BinaryTreeNode node3 = new BinaryTreeNode();
node3.value = 3;
BinaryTreeNode node4 = new BinaryTreeNode();
node4.value = 4;
BinaryTreeNode node5 = new BinaryTreeNode();
node5.value = 5;
node5.left = node4;
node4.left = node3;
node3.left = node2;
node2.left = node1;
System.out.print("Before convert: ");
printTree(node5);
System.out.println("null");
BinaryTreeNode head = convert(node5);
System.out.print("After convert : ");
printList(head);
System.out.println();
}
// 1
// \
// 2
// \
// 3
// \
// 4
// \
// 5
private static void test03() {
BinaryTreeNode node1 = new BinaryTreeNode();
node1.value = 1;
BinaryTreeNode node2 = new BinaryTreeNode();
node2.value = 2;
BinaryTreeNode node3 = new BinaryTreeNode();
node3.value = 3;
BinaryTreeNode node4 = new BinaryTreeNode();
node4.value = 4;
BinaryTreeNode node5 = new BinaryTreeNode();
node5.value = 5;
node1.right = node2;
node2.right = node3;
node3.right = node4;
node4.right = node5;
System.out.print("Before convert: ");
printTree(node1);
System.out.println("null");
BinaryTreeNode head = convert(node1);
System.out.print("After convert : ");
printList(head);
System.out.println();
}
// 只有一個(gè)結(jié)點(diǎn)
private static void test04() {
BinaryTreeNode node1 = new BinaryTreeNode();
node1.value = 1;
System.out.print("Before convert: ");
printTree(node1);
System.out.println("null");
BinaryTreeNode head = convert(node1);
System.out.print("After convert : ");
printList(head);
System.out.println();
}
// 沒(méi)有結(jié)點(diǎn)
private static void test05() {
System.out.print("Before convert: ");
printTree(null);
System.out.println("null");
BinaryTreeNode head = convert(null);
System.out.print("After convert : ");
printList(head);
System.out.println();
}
}
http://wiki.jikexueyuan.com/project/for-offer/images/40.png" alt="" />