LC.P897[递增顺序搜索树]

方法一：DFS

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/**
 * 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 TreeNode increasingBST(TreeNode root) {
        List<TreeNode> list = new ArrayList<>();
        inOrder(root, list);
        TreeNode ans = new TreeNode(-1), p = ans;
        for (TreeNode node : list) {
            p.right = node;
            p = node;
        }
        return ans.right;
    }

    private void inOrder(TreeNode root, List<TreeNode> list) {
        if (root == null) return;
        inOrder(root.left, list);
        list.add(new TreeNode(root.val));
        inOrder(root.right, list);
    }
}
  • 时间复杂度：$O(n)$
  • 空间复杂度：$O(n)$

方法二：栈

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/**
 * 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 TreeNode increasingBST(TreeNode root) {
        List<TreeNode> list = new ArrayList<>();
        Deque<TreeNode> stack = new ArrayDeque<>();
        while (root != null || !stack.isEmpty()) {
            while (root != null) {
                stack.push(root);
                root = root.left;
            }
            root = stack.pop();
            list.add(root);
            root = root.right;
        }
        TreeNode ans = new TreeNode(-1), p = ans;
        for (TreeNode node : list) {
            p.right = node;
            node.left = null;
            p = node;
        }
        return ans.right;
    }
}
  • 时间复杂度：$O(n)$
  • 空间复杂度：$O(n)$