非递归遍历二叉树-前序中序

思路：

前序遍历路径上节点的值先放入list，然后把节点的right-child放入栈，一次前序遍历到叶节点的时候，去栈中取出节点，循环进行前序遍历到叶节点。最后返回list

public class Solution {
    public ArrayList<Integer> preorderTraversal(TreeNode root) {
        ArrayList<Integer> result = new ArrayList<Integer>();
        
        Stack<TreeNode> stack = new Stack<TreeNode>();
        
        if(root == null) {
            return result;
        }
        TreeNode node = null;
        stack.add(root);
        while(!stack.empty()) {
            node  = stack.pop();
            while(node != null) {
                result.add(node.val);
                if(node.right != null) {
                    stack.push(node.right);
                }
                node = node.left;
            }
        }
        return result;
    }
}

 非递归中序遍历如下

public ArrayList<Integer> inorderTraversal(TreeNode root) {
        ArrayList<Integer> result = new ArrayList<Integer>();
        Stack<TreeNode> stack = new Stack<TreeNode>();
        if(root == null) {
            return result;
        }
        
        stack.add(root);
        TreeNode node = root.left;
        while(!stack.empty() || node != null) {  //第二个条件意义{1,#,2}
            while(node != null) {
                stack.add(node);
                node = node.left;  //左子树遍历到头，依次入栈
            }
            
            node = stack.pop();
            result.add(node.val);
            node = node.right;     //右子树遍历
        }
        return result;
    }

public class TreeNode {
    int val;
    TreeNode left;
    TreeNode right;
    TreeNode(int x) { val = x; }
}