從前序與中序（中序與後序）遍歷序列構造二叉樹

題目來源：LeetCode
根據一棵樹的前序遍歷與中序遍歷構造二叉樹。

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
class Solution {
    public TreeNode buildTree(int[] preorder, int[] inorder) {
        if(preorder.length == 0){        //判斷樹是否爲空
            return null;
        }
        int rootValue = preorder[0];     //根結點就是前序遍歷preorder的第一個元素
        int LeftCount;                   
        for(LeftCount = 0; LeftCount < inorder.length; LeftCount++){
            if(inorder[LeftCount] == rootValue){//在中序遍歷inorder中找到根結點
                break;
            }
        }
        TreeNode root = new TreeNode(rootValue);     //將根結點的值裝入結點
        //將preorder中的元素從preorder[1]到preorder[1 + LeftCount](左開右閉，不包括後者)拷貝到新的數組leftPreorder中
        int[] leftPreorder = Arrays.copyOfRange(preorder, 1, 1 + LeftCount);
        //將inorder中的元素從inorder[0]到inorder[LeftCount]拷貝到leftInorder中
        int[] leftInorder = Arrays.copyOfRange(inorder, 0, LeftCount);
        //遞歸
        root.left = buildTree(leftPreorder,leftInorder);
        //同理，寫出右子樹的過程
        int[] rightPreorder = Arrays.copyOfRange(preorder, LeftCount + 1, preorder.length);
        int[] rightInorder = Arrays.copyOfRange(inorder, LeftCount + 1, inorder.length);
        root.right = buildTree(rightPreorder,rightInorder);
        return root;
    }
}

同理可寫出從中序與後序遍歷序列構造二叉樹的代碼：

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
class Solution {
    public TreeNode buildTree(int[] inorder, int[] postorder) {
        if(inorder.length == 0){
            return null;
        } 
        int rootValue = postorder[postorder.length - 1];
        int LeftCount;
        for(LeftCount = 0; LeftCount < inorder.length; LeftCount++){
            if(inorder[LeftCount] == rootValue){
                break;
            }
        }
        TreeNode root = new TreeNode(rootValue);
        //這裏注意Arrays.copyOfRange()方法拷貝時的範圍是：左閉右開
        int[] leftInorder = Arrays.copyOfRange(inorder, 0, LeftCount);
        int[] leftPostorder = Arrays.copyOfRange(postorder, 0, LeftCount);
        root.left = buildTree(leftInorder, leftPostorder);
        int[] rightInorder = Arrays.copyOfRange(inorder, LeftCount + 1, postorder.length);
        int[] rightPostorder = Arrays.copyOfRange(postorder, LeftCount, postorder.length - 1);
        root.right = buildTree(rightInorder, rightPostorder);
        return root;
    }
}