【LeetCode】94. Binary Tree Inorder Traversal

Difficulty: Medium

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Description

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

Given a binary tree, return the inorder traversal of its nodes' values.

Example:

Input: [1,null,2,3]
   1
    \
     2
    /
   3

Output: [1,3,2]

Follow up: Recursive solution is trivial, could you do it iteratively?

Intuition

1. Recursion

2. Iteration

 

Solution

Recursion

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

　　

Iteration

    public List<Integer> inorderTraversal(TreeNode root) {
        List<Integer> list = new LinkedList<>();
        Stack<TreeNode> stk = new Stack<>();
        while(root!=null || !stk.isEmpty()){
            while(root!=null){
                stk.push(root);
                root=root.left;
            }
            list.add(stk.peek().val);
            root = stk.pop().right;
        }
        return list;
    }

　　

Complexity

Time complexity : O(n)

Space complexity : O(n)

 

 More:【目录】LeetCode Java实现