二叉树的遍历方式

先序遍历(Pre-Order Traversal)
指先访问根，然后访问子树的遍历方式
中序遍历(In-Order Traversal)
指先访问左（右）子树，然后访问根，最后访问右（左）子树的遍历方式
后序遍历(Post-Order Traversal)
指先访问子树，然后访问根的遍历方式

递归实现

递归遍历二叉树非常简单，这里不做赘述，先给出中序遍历的代码

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
public class Solution {
    public List<Integer> inorderTraversal(TreeNode root) {
        List<Integer> ans = new ArrayList<>();
        if(root != null) tra(root, ans);
        return ans;
    }
    
    private void tra(TreeNode node, List<Integer> ans){
        if(node.left != null) tra(node.left, ans);
        ans.add(node.val);
        if(node.right != null) tra(node.right, ans);
    }
}

先序遍历只需要将递归方法中的ans.add(node.val);放到最前面，后序遍历只需要将递归方法中的ans.add(node.val);放到最后面就行。

非递归实现

前序遍历

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
public class Solution {
    public List<Integer> preorderTraversal(TreeNode root) {
        //用来保存结果的ArrayList.
        List<Integer> ans = new ArrayList<>();
        //用来存储右边节点的栈
        Stack<TreeNode> rights = new Stack<>();
        //遍历到的当前的节点
        TreeNode node = root;
        while(node != null){
            //将当前节点的值先放入结果
            ans.add(node.val);
            //如果当前节点的右边子节点不为空，则将其压入栈
            if(node.right != null){
                rights.add(node.right);
            }
            //将当前节点置为当前节点的左节点
            node = node.left;
            //此时若为空，则前一步作废，将当前节点置为之前压入栈的右边的节点
            if(node == null && !rights.isEmpty()){
                node = rights.pop();
            }
        }
        return ans;
    }
}

中序遍历

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
public class Solution {
    public List<Integer> inorderTraversal(TreeNode root) {
        List<Integer> ans = new ArrayList<>();
        Stack<TreeNode> stack = new Stack<>();
        TreeNode node = root;
        while(node != null || !stack.empty()){
            while(node != null){
                stack.add(node);
                node = node.left;
            }
            node = stack.pop();
            ans.add(node.val);
            node = node.right;
        }
        return ans;
    }
    
}

后序遍历

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
public class Solution {
    public List<Integer> postorderTraversal(TreeNode root) {
       LinkedList<Integer> ans = new LinkedList<>();
       Stack<TreeNode> stack = new Stack<>();
       if(root == null) return ans;
       stack.add(root);
       while(!stack.empty()){
           TreeNode node = stack.pop();
           ans.addFirst(node.val);
           if(node.left != null) stack.add(node.left);
           if(node.right != null) stack.add(node.right);
       }
       return ans;
    }
}