【Java数据结构】非递归遍历二叉树

1. 定义二叉树结点类TreeNode

public class TreeNode {
    char val;
    TreeNode left;
    TreeNode right;
    public TreeNode(char val) {
        this.val = val;
    }
}

2. 前序遍历（非递归）

public class TestBinaryTree {
    //前序遍历-非递归
    public preOrderTraversalNor (TreeNode root) {
        if (root == null) return;
        Stack<TreeNode> stack = new Stack<>();
        TreeNode cur = root;

        while (cur != null || !stack.empty()) {
            while (cur != null) {
                stack.push(cur);
                System.out.println(cur.val + " ");
                cur = cur.left;
            }
            //当cur.left为空时，弹栈
            TreeNode top = stack.pop();
            cur = top.right;    //1.null  2.不是null的情况
        }
    }
}

3. 中序遍历（非递归）

    void inOrderTraversalNor (TreeNode root) {
        if (root == null) return;
        Stack<TreeNode> stack = new Stack<>();
        TreeNode cur = root;

        while (cur != null || !stack.empty()) {
            while (cur != null) {
                stack.push(cur);
                cur = cur.left;
            }
            //当cur==null，说明走到了二叉树的最左侧，开始弹栈并打印
            TreeNode top = stack.pop();
            System.out.print(top.val+" ");
            cur = top.right;  
        }
        System.out.println();
    }

4. 后序遍历（非递归）

主要注意解决cur当前结点被打印和 cur的右侧结点已被打印的问题

    void postOrderTraversalNor(TreeNode root) {
        if (root == null) return;
        Stack<TreeNode> stack = new Stack<>();
        TreeNode prev = null;
        TreeNode cur = root;        
        while ( cur != null || !stack.empty()) {
            while ( cur != null) {
                stack.push(cur);
                cur = cur.left;
            }
            //走到最左边了
            cur = stack.peek();
            if (cur.right == null || cur.right == prev) {    //cur.right == prev表明右结点被打印过了
                stack.pop();
                System.out.print(cur.val+" ");
                //记录被打印过的结点
                prev = cur;
                cur = null;    //打印完一定要置空
            } else {
                cur = cur.right;
            }
        }
    }

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