二叉樹相關的演算法，遍歷使用了遞迴和迭代兩種演算法，可作為結果對比。
理解起來不難，直接上程式碼，有空再補下注釋說明原理。

package com.junyang.algodemo.Tree;

import java.util.LinkedList;
import java.util.Stack;

public class TreeUtil {
    private static final int DEEP = 5;

    /* 建立一個深度為deep的滿二叉樹，目的是為驗證其它演算法 */
    public static TreeNode createTree(int deep) {
        /* 處理deep為0 和為1的特殊情況 */
 

        if (0 >= deep) {
            return null;
        }

        int count = 0;
        TreeNode root = new TreeNode(count++);
        if (1 == deep) {
            return root;
        }

        /* 滿二叉樹建立完成的條件為，當前這一層達到2的 deep-1 次方 */
        double maxLayerNum = Math.pow(2f, deep - 1f);

        TreeNode last = root;

        /* 通過佇列按層建立二叉樹 */
 

        LinkedList<TreeNode> queue = new LinkedList<>();
        queue.add(root);
        int width = 0;

        while (!queue.isEmpty()) {
            TreeNode temp = queue.remove();
            temp.left = new TreeNode(count++);
            temp.right = new TreeNode(count++);

            queue.add(temp.left);
            queue.add(temp.right);
            width += 2
 
;

            if (width == maxLayerNum) {
                return root;
            }

            if (temp == last) {
                last = last.right;
                width = 0;
            }
        }

        return root;
    }

    /* 按層遍歷同時按行顯示 */
    public static void printTreeForLayer(TreeNode root) {
        if (null == root) {
            return;
        }

        LinkedList<TreeNode> queue = new LinkedList<>();
        queue.add(root);
        TreeNode last = root;
        TreeNode nlast = root;
        int lineNum = 1;

        System.out.print("num[" + lineNum++ + "]:");
        while (!queue.isEmpty()) {
            TreeNode temp = queue.remove();

            if (null != temp.left) {
                queue.add(temp.left);
                nlast = temp.left;
            }

            if (null != temp.right) {
                queue.add(temp.right);
                nlast = temp.right;
            }

            System.out.print(temp.data + " ");
            if (temp == last) {
                System.out.println();
                if (!queue.isEmpty()) {
                    System.out.print("num[" + lineNum++ + "]:");
                }
                last = nlast;
            }
        }
    }

    /* 先序遍歷 —— 遞迴 */
    public static void printTreeForRootFirst1(TreeNode root) {
        if (root == null) {
            return;
        }

        System.out.print(root.data + " ");
        printTreeForRootFirst1(root.left);
        printTreeForRootFirst1(root.right);

    }

    /* 先序遍歷 —— 迭代*/
    public static void printTreeForRootFirst2(TreeNode root) {
        if (null == root) {
            return;
        }

        Stack<TreeNode> stack = new Stack<>();
        stack.push(root);

        while (!stack.isEmpty()) {
            TreeNode top = stack.pop();
            System.out.print(top.data + " ");

            if (top.right != null) {
                stack.push(top.right);
            }

            if (top.left != null) {
                stack.push(top.left);
            }
        }
    }


    /* 中序遍歷 —— 遞迴 */
    public static void printTreeForRootMiddle(TreeNode root) {
        if (root == null) {
            return;
        }

        printTreeForRootMiddle(root.left);
        System.out.print(root.data + " ");
        printTreeForRootMiddle(root.right);

    }

    /* 中序遍歷 —— 迭代 */
    public static void printTreeForRootMiddle2(TreeNode root) {
        if (root == null) {
            return;
        }

        Stack<TreeNode> stack = new Stack<>();

        TreeNode cur = root;
        while (!stack.isEmpty() || cur != null) {
            if (cur != null) {
                stack.push(cur);
                cur = cur.left;
            } else {
                cur = stack.pop();
                System.out.print(cur.data + " ");
                cur = cur.right;
            }
        }
    }

    /* 後序遍歷 —— 遞迴 */
    public static void printTreeForRootLast(TreeNode root) {
        if (root == null) {
            return;
        }

        printTreeForRootLast(root.left);
        printTreeForRootLast(root.right);
        System.out.print(root.data + " ");

    }

    /* 後序遍歷 —— 迭代 */
    public static void printTreeForRootLast2(TreeNode root) {
        if (root == null) {
            return;
        }

        Stack<TreeNode> stack = new Stack<>();
        Stack<TreeNode> output = new Stack<>();

        stack.push(root);
        while (!stack.isEmpty()) {
            TreeNode temp = stack.pop();
            output.push(temp);

            if (null != temp.left) {
                stack.push(temp.left);
            }

            if (null != temp.right) {
                stack.push(temp.right);
            }
        }

        while (!output.isEmpty()) {
            System.out.print(output.pop().data + " ");
        }
    }

    /* 獲取二叉樹的寬度 */
    public static int getWidth(TreeNode root) {
        if (null == root) {
            return 0;
        }

        int maxWidth = 1;
        int curWidth = 0;
        LinkedList<TreeNode> linkedList = new LinkedList<>();
        linkedList.add(root);

        TreeNode last = root;
        TreeNode nlast = root;
        while (!linkedList.isEmpty()) {
            TreeNode temp = linkedList.remove();

            if (null != temp.left) {
                linkedList.add(temp.left);
                nlast = temp.left;
                curWidth++;
            }

            if (null != temp.right) {
                linkedList.add(temp.right);
                nlast = temp.right;
                curWidth++;
            }

            if (temp == last) {
                last = nlast;
                maxWidth = maxWidth > curWidth ? maxWidth : curWidth;
                curWidth = 0;
            }
        }

        return maxWidth;
    }

    /* 獲取二叉樹的深度 */
    public static int getDeep(TreeNode root) {
        if (null == root) {
            return 0;
        }

        int leftDeep = 0;
        if (root.left != null) {
            leftDeep = getDeep(root.left);
        }

        int rightDeep = 0;
        if (root.right != null) {
            rightDeep = getDeep(root.right);
        }

        return (rightDeep > leftDeep ? rightDeep : leftDeep) + 1;
    }

    public static void main(String[] args) {
        TreeNode root = createTree(DEEP);
        System.out.println("按層遍歷");
        printTreeForLayer(root);

        System.out.println("先序遍歷");
        printTreeForRootFirst1(root);
        System.out.println();
        printTreeForRootFirst2(root);
        System.out.println();

        System.out.println("中序遍歷");
        printTreeForRootMiddle(root);
        System.out.println();
        printTreeForRootMiddle2(root);
        System.out.println();

        System.out.println("後序遍歷");
        printTreeForRootLast(root);
        System.out.println();
        printTreeForRootLast2(root);
        System.out.println();

        System.out.println("deep:" + getDeep(root));
        System.out.println("width:" + getWidth(root));
    }
}

輸出：

按層遍歷
num[1]:0 
num[2]:1 2 
num[3]:3 4 5 6 
num[4]:7 8 9 10 11 12 13 14 
num[5]:15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 
先序遍歷
0 1 3 7 15 16 8 17 18 4 9 19 20 10 21 22 2 5 11 23 24 12 25 26 6 13 27 28 14 29 30 
0 1 3 7 15 16 8 17 18 4 9 19 20 10 21 22 2 5 11 23 24 12 25 26 6 13 27 28 14 29 30 
中序遍歷
15 7 16 3 17 8 18 1 19 9 20 4 21 10 22 0 23 11 24 5 25 12 26 2 27 13 28 6 29 14 30 
15 7 16 3 17 8 18 1 19 9 20 4 21 10 22 0 23 11 24 5 25 12 26 2 27 13 28 6 29 14 30 
後序遍歷
15 16 7 17 18 8 3 19 20 9 21 22 10 4 1 23 24 11 25 26 12 5 27 28 13 29 30 14 6 2 0 
15 16 7 17 18 8 3 19 20 9 21 22 10 4 1 23 24 11 25 26 12 5 27 28 13 29 30 14 6 2 0 
deep:5
width:16