leetcode — binary-tree-inorder-traversal

import java.util.Arrays;
import java.util.Stack;
import java.util.TreeMap;

/**
 *
 * Source : https://oj.leetcode.com/problems/binary-tree-inorder-traversal/
 *
 *
 * Given a binary tree, return the inorder traversal of its nodes' values.
 *
 * For example:
 * Given binary tree {1,#,2,3},
 *
 *    1
 *     \
 *      2
 *     /
 *    3
 *
 * return [1,3,2].
 *
 * Note: Recursive solution is trivial, could you do it iteratively?
 *
 * confused what "{1,#,2,3}" means? > read more on how binary tree is serialized on OJ.
 *
 * OJ's Binary Tree Serialization:
 *
 * The serialization of a binary tree follows a level order traversal, where '#' signifies
 * a path terminator where no node exists below.
 *
 * Here's an example:
 *
 *    1
 *   / \
 *  2   3
 *     /
 *    4
 *     \
 *      5
 *
 * The above binary tree is serialized as "{1,2,3,#,#,4,#,#,5}".
 *
 */
public class BinaryTreeInOrderTraversal {
    private int[] result = null;
    int pos = 0;

    public int[] traversal(char[] tree) {
        result = new int[tree.length];
        pos = 0;
        traversalByRecursion(createTree(tree));
        return result;
    }

    public int[] traversal1(char[] tree) {
        result = new int[tree.length];
        pos = 0;
        traversalbyIterator(createTree(tree));
        return result;
    }

    /**
     * 对二叉树进行中序遍历
     *
     * 树的遍历分为：
     *  深度优先：
     *      先序遍历：先访问根节点然后依次访问左右子的节点
     *      中序遍历：先访问左子节点，然后访问根节点，在访问右子节点
     *      后序遍历：先访问左右子节点，然后访问根节点
     *
     * 先用递归实现中序遍历
     *
     * @param root
     * @return
     */
    public void traversalByRecursion(TreeNode root) {
        if (root == null) {
            return;
        }
        traversalByRecursion(root.leftChild);
        result[pos++] = root.value;
        traversalByRecursion(root.rightChild);
    }

    /**
     * 使用循环来进行中序遍历,借助栈实现
     *
     * @param root
     */
    public void traversalbyIterator (TreeNode root) {
        Stack<TreeNode> stack = new Stack<TreeNode>();
        TreeNode cur = root;
        while (cur != null || stack.size() > 0) {
            if (cur == null) {
                // 当前节点为空，表示已经是叶子节点，
                TreeNode node = stack.pop();
                result[pos++] = node.value;
                cur = node.rightChild;
            } else {
                stack.push(cur);
                cur = cur.leftChild;
            }
        }
    }

    public TreeNode createTree (char[] treeArr) {
        TreeNode[] tree = new TreeNode[treeArr.length];
        for (int i = 0; i < treeArr.length; i++) {
            if (treeArr[i] == '#') {
                tree[i] = null;
                continue;
            }
            tree[i] = new TreeNode(treeArr[i]-'0');
        }
        int pos = 0;
        for (int i = 0; i < treeArr.length && pos < treeArr.length-1; i++) {
            if (tree[i] != null) {
                tree[i].leftChild = tree[++pos];
                if (pos < treeArr.length-1) {
                    tree[i].rightChild = tree[++pos];
                }
            }
        }
        return tree[0];
    }

    private class TreeNode {
        TreeNode leftChild;
        TreeNode rightChild;
        int value;

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

        public TreeNode() {
        }
    }

    public static void main(String[] args) {
        BinaryTreeInOrderTraversal binaryTreeInOrderTraversal = new BinaryTreeInOrderTraversal();
        char[] arr1 = new char[]{'1','#','2','3'};
        char[] arr2 = new char[]{'1','2','3','#','#','4','#','#','5'};
        System.out.println(Arrays.toString(binaryTreeInOrderTraversal.traversal(arr1)));
        System.out.println(Arrays.toString(binaryTreeInOrderTraversal.traversal(arr2)));

        System.out.println(Arrays.toString(binaryTreeInOrderTraversal.traversal1(arr1)));
        System.out.println(Arrays.toString(binaryTreeInOrderTraversal.traversal1(arr2)));
    }
}