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

TreeNode() {
    }

TreeNode(int val) {
this.val = val;
    }

TreeNode(int val, TreeNode left, TreeNode right) {
this.val = val;
this.left = left;
this.right = right;
    }
}

class Solution {
public static void main(String[] args) {
TreeNode node5 = new TreeNode(5, null, null);
TreeNode node4 = new TreeNode(4, null, node5);
TreeNode node2 = new TreeNode(2, node4, null);
TreeNode node3 = new TreeNode(3, null, null);
TreeNode node1 = new TreeNode(1, node2, node3);
morrisPre(node1);
    }
}

题解

思路：同样使用栈，按照对应的遍历方式来将所有的节点存储，接着来进行依次出栈取值，这里比较细节的一点就是要出栈取值的节点后会跟随这一个null值用于进行标识。

前序遍历

//统一迭代：前序遍历
public List<Integer> preorderTraversal(TreeNode root) {
Deque<TreeNode> stack = new LinkedList<>();//根据指定的遍历顺序来进行存放
List<Integer> result = new ArrayList<>();
if(root != null){
stack.push(root);
    }
while(!stack.isEmpty()){
TreeNode node = stack.peek();
if(node != null){
stack.pop();
//前序遍历：在栈中存储方式为右-左-中
if(node.right != null){
stack.push(node.right);
            }
if(node.left != null){
stack.push(node.left);
            }
//null则为之后记录遍历值时的标识
stack.push(node);
stack.push(null);
        }else{
stack.pop();//先将null标识弹出，之后直接来统计拿到值
TreeNode valNode = stack.pop();
result.add(valNode.val);
        }
    }
return result;
}

中序遍历

//统一迭代：中序遍历
public List<Integer> postorderTraversal(TreeNode root) {
Deque<TreeNode> stack = new LinkedList<>();//根据指定的遍历顺序来进行存放
List<Integer> result = new ArrayList<>();
if(root != null){
stack.push(root);
    }
while(!stack.isEmpty()){
TreeNode node = stack.peek();
if(node != null){
stack.pop();
//中序遍历：在栈中存储方式为右-中-左
if(node.right != null){
stack.push(node.right);
            }
stack.push(node);
stack.push(null);
if(node.left != null){
stack.push(node.left);
            }

        }else{
stack.pop();//先将null标识弹出，之后直接来统计拿到值
TreeNode valNode = stack.pop();
result.add(valNode.val);
        }
    }
return result;
}

后序遍历

//统一迭代：后序遍历
public List<Integer> postorderTraversal(TreeNode root) {
Deque<TreeNode> stack = new LinkedList<>();//根据指定的遍历顺序来进行存放
List<Integer> result = new ArrayList<>();
if(root != null){
stack.push(root);
    }
while(!stack.isEmpty()){
TreeNode node = stack.peek();
if(node != null){
stack.pop();
stack.push(node);
stack.push(null);
//中序遍历：在栈中存储方式为右-中-左
if(node.right != null){
stack.push(node.right);
            }
if(node.left != null){
stack.push(node.left);
            }

        }else{
stack.pop();//先将null标识弹出，之后直接来统计拿到值
TreeNode valNode = stack.pop();
result.add(valNode.val);
        }
    }
return result;
}