二叉樹的遍歷方式

先序遍歷(Pre-Order Traversal)
指先訪問根，然后訪問子樹的遍歷方式
中序遍歷(In-Order Traversal)
指先訪問左（右）子樹，然后訪問根，最后訪問右（左）子樹的遍歷方式
后序遍歷(Post-Order Traversal)
指先訪問子樹，然后訪問根的遍歷方式

遞歸實現

遞歸遍歷二叉樹非常簡單，這里不做贅述，先給出中序遍歷的代碼

/**
 * 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);
    }
}

先序遍歷只需要將遞歸方法中的ans.add(node.val);放到最前面，后序遍歷只需要將遞歸方法中的ans.add(node.val);放到最后面就行。

非遞歸實現

前序遍歷

/**
 * 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) {
        //用來保存結果的ArrayList.
        List<Integer> ans = new ArrayList<>();
        //用來存儲右邊節點的棧
        Stack<TreeNode> rights = new Stack<>();
        //遍歷到的當前的節點
        TreeNode node = root;
        while(node != null){
            //將當前節點的值先放入結果
            ans.add(node.val);
            //如果當前節點的右邊子節點不為空，則將其壓入棧
            if(node.right != null){
                rights.add(node.right);
            }
            //將當前節點置為當前節點的左節點
            node = node.left;
            //此時若為空，則前一步作廢，將當前節點置為之前壓入棧的右邊的節點
            if(node == null && !rights.isEmpty()){
                node = rights.pop();
            }
        }
        return ans;
    }
}

中序遍歷

/**
 * 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;
    }
    
}

后序遍歷

/**
 * 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;
    }
}