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题目链接: https://leetcode.com/problems/binary-tree-inorder-traversal/

GitHub答案源代码请点击这里

Recursive解法

class Solution {
    public List<Integer> inorderTraversal(TreeNode root) {
        List<Integer> res = new ArrayList<>();
        dfs(root, res);
        return res;
    }
    
    private void dfs(TreeNode node, List<Integer> res) {
        if (node == null) {
            return;
        }
        
        dfs(node.left, res);
        res.add(node.val);
        dfs(node.right, res);
    }
}

Iterative解法 

class Solution {
    public List<Integer> inorderTraversal(TreeNode root) {
        if (root == null) {
            return new ArrayList<>();
        }
        
        Deque<TreeNode> stack = new ArrayDeque<>();
        List<Integer> res = new ArrayList<>();
        leftmost(root, stack);
        
        while (!stack.isEmpty()) {
            TreeNode cur = stack.pop();
            res.add(cur.val);
            
            leftmost(cur.right, stack);
        }
        
        return res;
    }
    
    private void leftmost(TreeNode node, Deque<TreeNode> stack) {
        while (node != null) {
            stack.push(node);
            node = node.left;
        }
    }
}