二叉树遍历的非递归

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前序遍历的非递归：1、在入栈时加入结果集，不停的取左子树入栈，直到为空。2、如果栈非空，pop栈顶结点，取其右子树作为当前结点，继续第一步，直到栈为空

中序遍历的非递归：1、在入栈时，不停的取左子树入栈，直到为空。2、如果栈非空，pop栈顶结点，加入结点集，取其右子树作为当前结点，继续第一步，直到栈为空

后序遍历的非递归：1、在遍历结点时，总是先将右子树结点入栈，再将左子树结点入栈。2、如果左子树结点和右子树结点为空或者右子树结点已经访问过，弹出栈顶元素，标记已访问结点，加入结果集，直到栈为空。

具体代码如下：

class TreeNode{    int val;    TreeNode left;    TreeNode right;    TreeNode(int x) { val = x; }}class Solution{    public List<Integer> preorderTraversal(TreeNode root)    {        List<Integer> ret = new ArrayList<Integer>();        Stack<TreeNode> stack = new Stack<TreeNode>();        TreeNode cur = root;        while(cur != null || !stack.empty())        {            while (cur != null)            {                ret.add(cur.val);                stack.push(cur);                cur = cur.left;            }            if (!stack.empty())            {                TreeNode tmp = stack.pop();                cur = tmp.right;            }        }        return ret;    }    public List<Integer> inorderTraversal(TreeNode root)    {        List<Integer> ret = new ArrayList<Integer>();        Stack<TreeNode> stack = new Stack<TreeNode>();        TreeNode cur = root;        while(cur != null || !stack.empty())        {            while (cur != null)            {                stack.push(cur);                cur = cur.left;            }            if (!stack.empty())            {                TreeNode tmp = stack.pop();                ret.add(tmp.val);                cur = tmp.right;            }        }        return ret;    }    public List<Integer> postorderTraversal(TreeNode root)    {        List<Integer> ret = new ArrayList<Integer>();        Stack<TreeNode> stack = new Stack<TreeNode>();        TreeNode cur = root, pre = null;        if (null == cur) return null;        stack.push(cur);        while (!stack.empty())        {            TreeNode tmp = stack.peek();            if ((tmp.left == null && tmp.right == null) || (pre != null && tmp.right == pre))            {                ret.add(tmp.val);                stack.pop();                pre = tmp;            }            else            {                if (tmp.right != null) stack.push(tmp.right);                if (tmp.left != null) stack.push(tmp.left);            }        }        return ret;    }}

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