构造数组的MaxTree（每日一道算法题）

题目：给一个数组（无重复的数字），构造一个二叉树，数组的每一个值对应一个二叉树节点。头结点是最大值。

package Tree;

import java.util.HashMap;
import java.util.Stack;

/**
 * @author:MindMrWang
 *2017年11月20日
 *:function:构造数组的MaxTree
 */
public class MaxTree {
    public static void main(String[] args) {
        int[]arr = {3,4,5,1,2};
        Node head = getMaxTree(arr);
        System.out.println(head.value);
        System.out.println(head.left.value);
        System.out.println(head.right.value);
        System.out.println(head.left.left.value);
        System.out.println(head.right.left.value);
    }


    public static Node getMaxTree(int[]arr) {
        Node []nArr = new Node[arr.length];
        for(int i=0;i<arr.length;i++) {
            nArr[i] = new Node(arr[i]);
        }
        Stack<Node> stack = new Stack<Node>();
        HashMap<Node,Node> lBigMap = new HashMap<Node,Node>();
        HashMap<Node,Node> rBigMap = new HashMap<Node,Node>();
        for(int i = 0;i!=nArr.length;i++) {
            Node curNode = nArr[i];
            while(!stack.isEmpty()&&stack.peek().value<curNode.value) {//如果stack不为空，且当前节点大于栈顶节点的值
                popStackSetMap(stack,lBigMap);//那么调用方法将栈顶的值设置为它的左边第一个最大值
            }
            stack.push(curNode);//每次都将节点数组向stack中push
        }
        while(!stack.isEmpty()) {
            popStackSetMap(stack,lBigMap);
        }


        for(int i = nArr.length-1;i!=-1;i--) {
            Node curNode = nArr[i];
            while(!stack.isEmpty()&&stack.peek().value<curNode.value) {//如果stack不为空，且当前节点x小于栈顶节点的值
                popStackSetMap(stack,rBigMap);//那么调用方法将栈顶的值设置为它的右边第一个最大值
            }
            stack.push(curNode);//每次都将节点数组向stack中push
        }
        while(!stack.isEmpty()) {
            popStackSetMap(stack,rBigMap);
        }

        Node head = null;


        for(int i = 0;i!=nArr.length;i++) {//根据上面的map，将这些节点进行关系关联成为一棵树
            Node curNode = nArr[i];
            Node left = lBigMap.get(curNode);
            Node right = rBigMap.get(curNode);
            if(left==null&&right==null) {
                head=curNode;
            } else if (left==null) {
                if (right.left==null) {
                    right.left = curNode;
                }else {
                    right.right = curNode;
                }
            }else if (right==null) {
                if(left.left==null) {
                    left.left = curNode;
                }else {
                    left.right = curNode;
                }
            }else {
                Node parent = left.value < right.value ? left:right;
                if(parent.left == null) {
                    parent.left = curNode;
                }else {
                    parent.right = curNode;
                }
            }
        }
        return head;

    }


    public static void popStackSetMap(Stack<Node> stack,HashMap<Node,Node> map) {
        Node popNode = stack.pop();
        if(stack.isEmpty()) {
            map.put(popNode, null);
        }else {
            map.put(popNode, stack.peek());
        }
    }

}




//节点类
class Node {
    public int value;
    public Node left;
    public Node right;
    public Node(int data) {
        this.value=data;
    }
}