Leetcode----<Convert Sorted Array to Binary Search Tree>

题目描述：

题解：

public class ConvertSortedArrayToBinarySearchTree {

    /**
     * hint: 数组已经排序，用类似二分的手段将数组分开建树，最后的高度差一定小于等于一，因为左右子树最多只会相差一个元素
     * @param nums
     * @return
     */
    public TreeNode sortedArrayToBST2(int[] nums) {
        if (nums == null || nums.length < 1) {
            return null;
        } else if (nums.length==1) {
            return new TreeNode(nums[0]);
        } else {
            return buildTree(nums,0,nums.length-1);
        }
    }

    private TreeNode buildTree(int[] nums, int left, int right) {
        if (left == right) {
            return new TreeNode(nums[left]);
        }
        if (left > right) return null;
        int mid = left + ((right-left)>>1);
        TreeNode root = new TreeNode(nums[mid]);
        TreeNode leftTree = buildTree(nums, left, mid - 1);
        TreeNode rightTree = buildTree(nums, mid + 1, right);
        root.left = leftTree;
        root.right = rightTree;
        return root;
    }


    /**
     * 失败解法：边建树，边调整，最后生成平衡树，但是平衡树的调整很麻烦
     * @param nums
     * @return
     */
    public TreeNode sortedArrayToBST(int[] nums) {
        if (nums == null || nums.length < 1) {
            return null;
        } else if (nums.length==1) {
            return new TreeNode(nums[0]);
        } else {
            TreeNode root = new TreeNode(nums[0]);
            int len = nums.length;
            for (int i = 1; i < len; i++) {
                addElement(nums[i],root);
                adjustTree(root);

            }

            return root;
        }
    }

    private void adjustTree(TreeNode root) {
        if (Math.abs(getHeight(root.left)-getHeight(root.right)) > 1) {
            // 调整二叉树 -- 不会写
        }
    }

    // 获取二叉树的高度
    private int getHeight(TreeNode root) {
        if (root == null) {
            return 0;
        }
        int heightOfLeft = getHeight(root.left);
        int heightOfRight = getHeight(root.right);
        return Math.max(heightOfLeft,heightOfRight) + 1;

    }

    private void addElement(int num, TreeNode root) {
        if (num >= root.val) {
            if (root.right == null) {
                root.right = new TreeNode(num);
                return;
            } else {
                addElement(num,root.right);
            }
        } else  {
            if (root.left == null) {
                root.left = new TreeNode(num);
                return;
            } else {
                addElement(num,root.left);
            }
        }
    }
}