二叉树的前中后序遍历，递归+迭代

前序遍历

递归

/**
 * 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> res = new ArrayList<Integer>();
        preorder(root,res);
        return res;
    }
    public void preorder(TreeNode root,List<Integer> res){
        if(root == null){
            return;
        } 
        res.add(root.val);//根
        preorder(root.left,res);//左
        preorder(root.right,res);//右
    }
    
}

迭代

/**
 * 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) {
        ArrayList<Integer> list = new ArrayList<>();
        Stack<TreeNode> stack = new Stack<>();
        while(!stack.isEmpty() || root != null){
            while(root != null){
                stack.push(root);
                list.add(root.val);
                root = root.left;
            }
            root = stack.pop();
            root = root.right;
        }
        return list;
    }
}

中序遍历

递归

/**
 * 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) {
        ArrayList<Integer> list = new ArrayList<>();
        inorder(root,list);
        return list;
    }
    public void inorder(TreeNode node, ArrayList list){
        if(node==null){
            return ;
        }
        inorder(node.left,list);
        list.add(node.val);
        inorder(node.right,list);
    }
}

迭代

/**
 * 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) {
        ArrayList<Integer> list = new ArrayList<>();
        Stack<TreeNode> stack = new Stack<>();
        while(root != null || !stack.isEmpty()){
            while(root != null){
                stack.push(root);
                root = root.left;
            }
            root = stack.pop();
            list.add(root.val);
            root = root.right;     
        }
        return list;
    }
}

后序遍历

递归

/**
 * 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> res = new ArrayList<Integer>();
        postorder(root,res);
        return res;
    }
    public void postorder(TreeNode root,List<Integer> res){
        if(root == null){
            return;
        }
        postorder(root.left,res);//左
        postorder(root.right,res);//右
        res.add(root.val);//根
    }
}

迭代

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


        }
        return list;
    }
}