LC.P1038[从二叉搜索树到更大和树]

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 {
    
    int sum;

    public TreeNode bstToGst(TreeNode root) {
        dfs(root);
        return root;
    }

    /**
     * 反序进行中序遍历：右->中->左，即单调递减
     */
    private void dfs(TreeNode node) {
        if (node == null) return;
        dfs(node.right);
        sum += node.val;
        node.val = sum;
        dfs(node.left);
    }
}
  • 时间复杂度：$O(n)$
  • 空间复杂度：$O(n)$