转载声明：文章来源https://blog.csdn.net/wayne566/article/details/79106372

列举了二叉树的前序、中序、后序的递归和非递归遍历方法，以及层次遍历、分层输出的层次遍历方法。

举例如下：

import java.util.LinkedList;
import java.util.List;
import java.util.Queue;
import java.util.Stack;

public class Main {
    public static void main(String[] args) {
        //构造树结构测试用
        TreeNode a = new TreeNode(1);
        TreeNode b = new TreeNode(2);
        TreeNode c = new TreeNode(3);
        TreeNode d = new TreeNode(4);
        TreeNode e = new TreeNode(5);
        TreeNode f = new TreeNode(6);
        TreeNode g = new TreeNode(7);
        a.left = b;
        a.right = c;
        b.right = d;
        c.left = e;
        c.right = f;
        f.left = g;
        System.out.print("recursivePreOrder: ");
        recursivePreOrder(a);
        System.out.print('\n' + "recursiveInOrder: ");
        recursiveInOrder(a);
        System.out.print('\n' + "recursivePostOrder: ");
        recursivePostOrder(a);
        System.out.print('\n' + "iterativePreOrder: ");
        iterativePreOrder(a);
        System.out.print('\n' + "iterativePreOrder_2: ");
        iterativePreOrder_2(a);
        System.out.print('\n' + "iterativeInOrder: ");
        iterativeInOrder(a);
        System.out.print('\n' + "iterativePostOrder: ");
        iterativePostOrder(a);
        System.out.print('\n' + "iterativePostOrder_2: ");
        iterativePostOrder_2(a);
        System.out.print('\n' + "iterativePostOrder_3: ");
        iterativePostOrder_3(a);
        System.out.print('\n' + "iterativeLevelOrder: ");
        iterativeLevelOrder(a);
        System.out.print('\n' + "iterativeLevelOrder_2: " + '\n');
        iterativeLevelOrder_2(a);
        System.out.print('\n' + "recursiveLevelOrder: ");
        recurLevelOrder(a);
        System.out.print('\n' + "recursiveLevelOrderBottom: " + '\n');
        List<List<Integer>> lists = recursiveLevelOrderBottom(a);
        for (List<Integer> list : lists) {
            for (int p : list) {
                System.out.print(p + " ");
            }
            System.out.println();
        }
    }


    public static void visit(TreeNode p) {
        System.out.print(p.val + " ");
    }


    //**********递归的先序遍历**********
    public static void recursivePreOrder(TreeNode p) {
        if (p == null) return;
        visit(p);
        recursivePreOrder(p.left);
        recursivePreOrder(p.right);
    }


    //**********递归的中序遍历**********
    public static void recursiveInOrder(TreeNode p) {
        if (p == null) return;
        recursiveInOrder(p.left);
        visit(p);
        recursiveInOrder(p.right);
    }


    //**********递归的后序遍历**********
    public static void recursivePostOrder(TreeNode p) {
        if (p == null) return;
        recursivePostOrder(p.left);
        recursivePostOrder(p.right);
        visit(p);
    }


    //**********非递归的先序遍历**********
    //手算的思想，先变访问边找，找到最左下方的，然后向上再向访问右边的
    public static void iterativePreOrder(TreeNode p) {
        if (p == null) return;
        Stack<TreeNode> stack = new Stack<TreeNode>();
        while (!stack.empty() || p != null) {
            while (p != null) {
                visit(p);
                stack.push(p);
                p = p.left;
            }
            p = stack.pop();
            p = p.right;
        }
    }

    //**********非递归的先序遍历**********
    //栈的思想，按层次倒着进栈，利用后进先出解决顺序问题
    public static void iterativePreOrder_2(TreeNode p) {
        if (p == null) return;
        Stack<TreeNode> stack = new Stack<TreeNode>();
        stack.push(p);
        while (!stack.empty()) {
            p = stack.pop();
            visit(p);
            if (p.right != null) stack.push(p.right);
            if (p.left != null) stack.push(p.left);
        }
    }


    //**********非递归的中序遍历**********
    public static void iterativeInOrder(TreeNode p) {
        if (p == null) return;
        Stack<TreeNode> stack = new Stack<TreeNode>();
        while (!stack.empty() || p != null) {
            while (p != null) {
                stack.push(p);
                p = p.left;
            }
            p = stack.pop();
            visit(p);
            p = p.right;
        }
    }


    //**********非递归的后序遍历**********
    //注意prev的作用
    public static void iterativePostOrder(TreeNode p) {
        if (p == null) return;
        Stack<TreeNode> stack = new Stack<TreeNode>();
        TreeNode prev = p;
        while (!stack.empty() || p != null) {
            while (p != null) {
                stack.push(p);
                p = p.left;
            }
            p = stack.peek().right;
            if (p == null || p == prev) {
                //若栈顶节点的右节点为空或者已经visit过，则按顺序应该访问栈顶节点
                p = stack.pop();
                visit(p);
                //prev用来标记已经visit过这个节点
                prev = p;
                p = null;
            }
        }
    }


    //**********非递归的后序遍历**********
    //和上一种方法思想类似
    public static void iterativePostOrder_2(TreeNode p) {
        if (p == null) return;
        Stack<TreeNode> stack = new Stack<TreeNode>();
        TreeNode prev = p;
        while (p != null) {
            while (p.left != null) {
                stack.push(p);
                p = p.left;
            }
            while (p != null && (p.right == null || p.right == prev)) {
                visit(p);
                prev = p;
                if (stack.empty()) return;
                p = stack.pop();
            }
            stack.push(p);
            p = p.right;
        }
    }


    //**********非递归的后序遍历**********
    //双栈法，易于理解
    public static void iterativePostOrder_3(TreeNode p) {
        if (p == null) return;
        Stack<TreeNode> stack = new Stack<TreeNode>();
        Stack<TreeNode> result = new Stack<TreeNode>();
        while (!stack.empty() || p != null) {
            while (p != null) {
                stack.push(p);
                result.push(p);
                p = p.right;
            }
            if (!stack.empty()) p = stack.pop().left;
        }
        while (!result.empty()) {
            p = result.pop();
            visit(p);
        }
    }


    //**********非递归的层次遍历**********
    public static void iterativeLevelOrder(TreeNode p) {
        if (p == null) return;
        LinkedList<TreeNode> queue = new LinkedList<TreeNode>();
        queue.offer(p);
        while (!queue.isEmpty()) {
            p = queue.poll();
            if (p.left != null) queue.offer(p.left);
            if (p.right != null) queue.offer(p.right);
            visit(p);
        }
    }


    //**********非递归的分层输出的层次遍历**********
    public static void iterativeLevelOrder_1(TreeNode p) {
        if (p == null) return;
        Queue<TreeNode> queue = new LinkedList<TreeNode>();
        queue.offer(p);
        while (!queue.isEmpty()) {
            int levelNum = queue.size();
            for (int i = 0; i < levelNum; i++) {
                p = queue.poll();
                if (p.left != null) queue.offer(p.left);
                if (p.right != null) queue.offer(p.right);
                visit(p);
            }
            System.out.println();
        }
    }


    //**********非递归的分层输出的层次遍历**********
    //维护两个int，代表上一层和下一层的节点数量，上一层遍历结束之后lineUp = lineDown; lineDown = 0;
    public static void iterativeLevelOrder_2(TreeNode p) {
        if (p == null) return;
        LinkedList<TreeNode> queue = new LinkedList<TreeNode>();
        int lineUp = 1, lineDown = 0;
        queue.offer(p);
        while (!queue.isEmpty()) {
            p = queue.poll();
            visit(p);
            if (p.left != null){
                queue.offer(p.left);
                lineDown++;
            }
            if (p.right != null){
                queue.offer(p.right);
                lineDown++;
            }
            if (--lineUp == 0) {
                lineUp = lineDown;
                lineDown = 0;
                System.out.println();
            }
        }
    }


    //**********递归的层次遍历访问**********
    public static void recurLevelOrder(TreeNode root) {
        if (root == null) return;
        int depth = maxDepth(root);
        //如果要倒序访问只需修改此处顺序
        for (int i = 1; i <= depth; i++) visitNodeAtDepth(root, i);
    }
    //访问特定层的节点
    public static void visitNodeAtDepth(TreeNode p, int depth) {
        if (p == null || depth < 1) return;
        //因为要按顺序访问（打印），所以要规定必须到某一层才能visit
        if (depth == 1) {
            visit(p);
            return;
        }
        //每次都要遍历depth之上的所有层
        visitNodeAtDepth(p.left, depth - 1);
        visitNodeAtDepth(p.right, depth - 1);
    }
    //得到树的层数
    public static int maxDepth(TreeNode root) {
        if (root == null) return 0;
        return Math.max(maxDepth(root.left), maxDepth(root.right)) + 1;
    }


    //**********递归的倒序层次遍历并保存结果至list**********
    //LeetCode107
    //之所以用LinkedList是因为有addFirst()方法，可以逆序保存
    public static List<List<Integer>> recursiveLevelOrderBottom(TreeNode root) {
        LinkedList<List<Integer>> lists = new LinkedList<List<Integer>>();
        addToList(lists, root, 1);
        return lists;
    }
    //将depth层的p节点保存至list
    public static void addToList(LinkedList<List<Integer>> lists, TreeNode p, int depth) {
        if (p == null) return;
        if (lists.size() < depth) lists.addFirst(new LinkedList<Integer>());
        //由于不用输出只是保存，可以使用get控制保存在哪一层，所以不用规定层数
        lists.get(lists.size() - depth).add(p.val);
        addToList(lists, p.left, depth + 1);
        addToList(lists, p.right, depth + 1);
    }
}

运行结果：

recursivePreOrder: 1 2 4 3 5 6 7 
recursiveInOrder: 2 4 1 5 3 7 6 
recursivePostOrder: 4 2 5 7 6 3 1 
iterativePreOrder: 1 2 4 3 5 6 7 
iterativePreOrder_2: 1 2 4 3 5 6 7 
iterativeInOrder: 2 4 1 5 3 7 6 
iterativePostOrder: 4 2 5 7 6 3 1 
iterativePostOrder_2: 4 2 5 7 6 3 1 
iterativePostOrder_3: 4 2 5 7 6 3 1 
iterativeLevelOrder: 1 2 3 4 5 6 7 
iterativeLevelOrder_2: 
1 
2 3 
4 5 6 
7 

recursiveLevelOrder: 1 2 3 4 5 6 7 
recursiveLevelOrderBottom: 
7 
4 5 6 
2 3 
1