题目
给出一个无重复元素的数组,构造此数组的MaxTree,
java代码
/**
* @Description: 构造数组的MaxTree
* @Author: lizhouwei
* @CreateDate: 2018/4/5 22:16
* @Modify by:
* @ModifyDate:
*/
public class Chapter1_8 {
public Node getMaxTree(int[] arr) {
if (arr == null) {
return null;
}
int length = arr.length;
Node[] nodes = new Node[length];
//初始化Node数组
for (int i = 0; i < length; i++) {
nodes[i] = new Node(arr[i]);
}
//存放当前节点往左 第一个大于当前节点的节点
Map<Node, Node> leftMap = new HashMap<Node, Node>();
//存放当前节点往右 第一个大于当前节点的节点
Map<Node, Node> rightMap = new HashMap<Node, Node>();
//存放遍历到节点
Stack<Node> stack = new Stack<Node>();
for (int i = 0; i < length; i++) {
//关键地方,判断栈顶元素的值是否小于当前元素的值,如果小于则在map中记录栈顶元素的往左数第一个元素
while (!stack.isEmpty() && stack.peek().value < nodes[i].value) {
popStackToMap(stack, leftMap);
}
//存放当前元素,前提保证栈中没有比小的元素
stack.push(nodes[i]);
}
//如果栈中还有元素,则循环获取栈中元素的往左数第一个大于他的元素
while (!stack.isEmpty()) {
popStackToMap(stack, leftMap);
}
for (int i = length - 1; i >= 0; i--) {
//关键地方,判断栈顶元素的值是否小于当前元素的值,如果小于则在map中记录栈顶元素的往右数第一个元素
while (!stack.isEmpty() && stack.peek().value < nodes[i].value) {
popStackToMap(stack, rightMap);
}
//存放当前元素,前提保证栈中没有比小的元素
stack.push(nodes[i]);
}
//如果栈中还有元素,则循环获取栈中元素的往右数第一个大于他的元素
while (!stack.isEmpty()) {
popStackToMap(stack, rightMap);
}
//再将 每个元素 左右 最大的元找见后,开始构造MaxTree
Node head = null;
for (int i = 0; i < length; i++) {
Node node = nodes[i];
Node leftNode = leftMap.get(node);
Node rightNode = rightMap.get(node);
//左右最大的元素都为空,则此元素是数组中最大,作为根节点
if (leftNode == null && rightNode == null) {
head = node;
} else if (leftNode == null) {
if (rightNode.left == null) {
rightNode.left = node;
} else {
rightNode.right = node;
}
} else if (rightNode == null) {
if (leftNode.left == null) {
leftNode.left = node;
} else {
leftNode.right = node;
}
} else {
Node parent = leftNode.value < rightNode.value ? leftNode : rightNode;
if (parent.left == null) {
parent.left = node;
} else {
parent.right = node;
}
}
}
return head;
}
public void popStackToMap(Stack<Node> stack, Map<Node, Node> map) {
Node node = stack.pop();
if (stack.isEmpty()) {
map.put(node, null);
} else {
map.put(node, stack.peek());
}
}
//简单的用递归中序遍历一下生成的MaxTree
public void recInOrder(Node head) {
if (head == null) {
return;
}
recInOrder(head.left);
System.out.print(head.value + " ");
recInOrder(head.right);
}
//测试
public static void main(String[] args) {
Chapter1_8 chapter = new Chapter1_8();
int[] arr = {3, 4, 5, 1, 2};
Node head = chapter.getMaxTree(arr);
chapter.recInOrder(head);
}
}
class Node {
public int value;
public Node left;
public Node right;
public Node(int value) {
this.value = value;
}
}