leetcode — convert-sorted-array-to-binary-search-tree

import java.util.ArrayList;
import java.util.Arrays;
import java.util.List;

/**
 *
 * Source : https://oj.leetcode.com/problems/convert-sorted-array-to-binary-search-tree/
 *
 * Given an array where elements are sorted in ascending order, convert it to a height balanced BST.
 *
 */
public class ConvertSortedArray {


    /**
     * 把一个已排序的数组转化我一颗高度平衡二叉搜索树，即AVL树，具有以下性质：
     * 它是一棵空树或者左右两个子树的高度差不超过1，并且左右子树也都是AVL树
     *
     * 因为是已排序的数组，只需要找到数组中间位置的值作为根节点，左边的为左子树，右边的为右子树，递归构造即可
     *
     * @param arr
     * @param left
     * @param right
     * @return
     */
    public TreeNode convert (int[] arr, int left, int right) {
        if (left > right) {
            return null;
        }
        int mid = (left + right) / 2;
        TreeNode root = new TreeNode(arr[mid]);
        root.leftChild = convert(arr, left, mid-1);
        root.rightChild = convert(arr, mid+1, right);
        return root;
    }


    public void binarySearchTreeToArray (TreeNode root, List<Character> chs) {
        if (root == null) {
            chs.add('#');
            return;
        }
        List<TreeNode> list = new ArrayList<TreeNode>();
        int head = 0;
        int tail = 0;
        list.add(root);
        chs.add((char) (root.value + '0'));
        tail ++;
        TreeNode temp = null;

        while (head < tail) {
            temp = list.get(head);
            if (temp.leftChild != null) {
                list.add(temp.leftChild);
                chs.add((char) (temp.leftChild.value + '0'));
                tail ++;
            } else {
                chs.add('#');
            }
            if (temp.rightChild != null) {
                list.add(temp.rightChild);
                chs.add((char)(temp.rightChild.value + '0'));
                tail ++;
            } else {
                chs.add('#');
            }
            head ++;
        }

        //去除最后不必要的
        for (int i = chs.size()-1; i > 0; i--) {
            if (chs.get(i) != '#') {
                break;
            }
            chs.remove(i);
        }
    }
    private class TreeNode {
        TreeNode leftChild;
        TreeNode rightChild;
        int value;

        public TreeNode(int value) {
            this.value = value;
        }

        public TreeNode() {

        }
    }

    public static void main(String[] args) {
        ConvertSortedArray convertSortedArray = new ConvertSortedArray();
        int[] arr = new int[]{1,2,3,4,5};
        List<Character> chs = new ArrayList<Character>();
        convertSortedArray.binarySearchTreeToArray(convertSortedArray.convert(arr, 0, arr.length-1), chs);
        System.out.println(Arrays.toString(chs.toArray(new Character[chs.size()])));
    }
}