二叉树非递归遍历

public class BiTreeTraversal {

    static class TreeNode {
        int val;
        TreeNode left;
        TreeNode right;
    }

    //先序遍历
    void preOrder(TreeNode root) {
        if (root == null) return;
        Stack stack = new Stack<>();
        while (!stack.isEmpty() || root != null) {
            //先遍历后入栈
            while (root != null) {
                System.out.print(root.val + " ");
                stack.push(root);
                root = root.left;
            }
            //根和左孩子都遍历完了，则更新root为当前节点的右孩子
            if (!stack.isEmpty()) {
                TreeNode t = stack.pop();
                root = t.right;
            }
        }
    }

    //中序遍历
    void inOrder(TreeNode root) {
        if (root == null) return;
        Stack stack = new Stack<>();
        while (!stack.isEmpty() || root != null) {
            //先把左孩子全部入栈
            while (root != null) {
                stack.push(root);
                root = root.left;
            }
            //出栈后就可遍历
            if (!stack.isEmpty()) {
                TreeNode t = stack.pop();
                System.out.print(t.val + " ");
                root = t.right;
            }
        }
    }

    //后序遍历
    void postOrder(TreeNode root) {
        if (root == null) return;
        Stack stack = new Stack<>();
        TreeNode last = null;
        while (!stack.isEmpty() || root != null) {
            while (root != null) {
                stack.push(root);
                root = root.left;
            }
            if (!stack.isEmpty()) {
                TreeNode t = stack.pop();
                //当前节点的右孩子为空，或右孩子已访问过
                if (t.right == null || last == t.right) {
                    System.out.print(t.val + " ");
                    last = t;
                } else {
                    //因为当前结点未打印，所以要重新放回去，等右孩子打印完之后回来打印
                    stack.push(t);
                    root = t.right;
                }
            }
        }
    }
}