144. 二叉树的前序遍历 - 力扣（LeetCode）

/**
 * Definition for a binary tree node.
 * public 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 List<Integer> preorderTraversal(TreeNode root) {
        List<Integer> result = new ArrayList();
        traversal(root,result);

        return result;
        
    }

    private void traversal(TreeNode root,List<Integer> result){
        if(root == null){
            return;
        }

        result.add(root.val);
        traversal(root.left,result);
        traversal(root.right,result);
    }
}

/**
 * Definition for a binary tree node.
 * public 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 List<Integer> preorderTraversal(TreeNode root) {
        List<Integer> result = new ArrayList();
        if(root == null){
            return result;
        }
        
        Stack<TreeNode> stack = new Stack<>();
        stack.push(root);
        while(!stack.isEmpty()){
            TreeNode pop = stack.pop();
            result.add(pop.val);
            if(pop.right != null){
                stack.push(pop.right);
            }
            if(pop.left != null){
                stack.push(pop.left);
            }
        }
        
        return result;
    }
}

145. 二叉树的后序遍历 - 力扣（LeetCode）

/**
 * Definition for a binary tree node.
 * public 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 List<Integer> postorderTraversal(TreeNode root) {
        List<Integer> result = new ArrayList<>();

        traversal(root,result);

        return result;
    }

    private void traversal(TreeNode root,List<Integer> result){
        if(root == null){
            return;
        }

        traversal(root.left,result);
        traversal(root.right,result);

        result.add(root.val);
    }
}

/**
 * Definition for a binary tree node.
 * public 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 List<Integer> postorderTraversal(TreeNode root) {
        List<Integer> result = new ArrayList<>();
        if(root == null){
            return result;
        }
        
        Stack<TreeNode> stack = new Stack<>();
        stack.push(root);
        while (!stack.isEmpty()){
            TreeNode pop = stack.pop();
            result.add(pop.val);
            if(pop.left != null){
                stack.push(pop.left);
            }
            if(pop.right != null){
                stack.push(pop.right);
            }
        }
        
        Collections.reverse(result);
        return result;
    }
}

94. 二叉树的中序遍历 - 力扣（LeetCode）

/**
 * Definition for a binary tree node.
 * public 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 List<Integer> inorderTraversal(TreeNode root) {
        List<Integer> result = new ArrayList<>();

        traversal(root,result);

        return result;
    }

    private void traversal(TreeNode root,List<Integer> result){
        if(root == null){
            return;
        }

        traversal(root.left,result);
        result.add(root.val);
        traversal(root.right,result);
    }
}

/**
 * Definition for a binary tree node.
 * public 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 List<Integer> inorderTraversal(TreeNode root) {
        if(root == null){
            return new ArrayList<>();
        }

        List<Integer> result = new ArrayList<>();
        Stack<TreeNode> stack = new Stack<>();
        while(root != null || !stack.isEmpty()){
            if(root != null){
                stack.push(root);
                root = root.left;
            }else{
                root = stack.pop();
                result.add(root.val);
                root = root.right;
            }
        }

        return result;
    }
}