面試題 24: 二叉搜索樹的後序遍歷序列

一. 題目

輸入一個整數數組,判斷該數組是不是某二叉搜索樹的後序遍歷的結果.如果是則返回true,否則返回false.假設輸入的數組的任意兩個數字都互不相同.

代碼請到我的代碼庫中下載 Point2Offer

二. 代碼

package week_5;

/**難度係數:***
 * 劍指offer: 二叉搜索樹的後序遍歷序列
 * 方法: 熟悉二叉搜索樹的概念,左子樹所有結點小於根節點,右子樹所有結點都大於根節點,左右子樹仍是二叉搜索樹,鍵值都不相等
 * 測試用例:
 * @author dingding
 * Date:2017-7-11 21:40
 * Declaration: All Rights Reserved!
 */
public class No24 {

    public static void main(String[] args) {
        test1();
        test2();
        test3();
    }

    //solution
    public static boolean verifyArrayIsBST(int[] sequence) {
        // 輸入的數組不能爲空，並且有數據
        if (sequence == null || sequence.length <= 0) {
            return false;
        }

        // 有數據，就調用輔助方法
        return verifyArrayIsBST(sequence, 0, sequence.length - 1);
    }

    private static boolean verifyArrayIsBST(int[] sequence,int start,int end){
        if (start >= end) {
            return true;
        }

        // 從左向右找第一個不大於根結點（sequence[end]）的元素的位置
        int index = start;
        while (index < end - 1 && sequence[index] < sequence[end]) {
            index++;
        }

        //搜索右子樹的結點大於根節點
        int right = index;
        while (right < end - 1 && sequence[right] > sequence[end]) {
            right++;
        }

        if (right != end - 1) {
            return false;
        }

        //遞歸,判斷左右子樹是不是BST
        return verifyArrayIsBST(sequence, start, index - 1) && verifyArrayIsBST(sequence, index, end - 1);
    }

    /*=======================測試用例===============*/
    private static void test1() {
        int[] sequence = {5,7,6,9,11,10,8};
        boolean result = verifyArrayIsBST(sequence);
        if (result) {
            System.out.println("YES!");
        }else {
            System.out.println("NO!");
        }
    }

    private static void test2() {
        int[] sequence = {7,4,6,5};
        boolean result = verifyArrayIsBST(sequence);
        if (result) {
            System.out.println("YES!");
        }else {
            System.out.println("NO!");
        }
    }

    private static void test3() {
        int[] data2 = {4, 6, 7, 5};
        System.out.println("true: " + verifyArrayIsBST(data2));
    }

}

---

有不妥當之處,麻煩告知:D