LeetCode.938.Range Sum of BST 二叉搜索树BST的范围和

题目描述

938 Range Sum of BST https://leetcode-cn.com/problems/range-sum-of-bst/

Given the root node of a binary search tree, return the sum of values of all nodes with value between L and R (inclusive).

The binary search tree is guaranteed to have unique values.

Example 1:

Input: root = [10,5,15,3,7,null,18], L = 7, R = 15
Output: 32

Example 2:

Input: root = [10,5,15,3,7,13,18,1,null,6], L = 6, R = 10
Output: 23

  Note: The number of nodes in the tree is at most 10000. The final answer is guaranteed to be less than 2^31.

解题过程

递归深度优先搜索 + 利用BST的性质剪枝 时间复杂度 O(n)，空间复杂度 O(n)

private static class SolutionV2020 {
    public int rangeSumBST(TreeNode root, int L, int R) {
        return dfs(root, L, R);
    }

    // 递归遍历二叉树，返回root中在[L,R]之间的结点和
    private int dfs(TreeNode root, int L, int R) {
        if (null == root) {
            return 0;
        }
        // 剪枝，剪掉根和右子树
        if (root.val > R) {
            return dfs(root.left, L, R);
        }
        // 剪枝，剪掉根和左子树
        if (root.val < L) {
            return dfs(root.right, L, R);
        }
        // 根在[L,R]之间
        return root.val + dfs(root.left, L, R) + dfs(root.right, L, R);
    }
}

GitHub代码

algorithms/leetcode/leetcode/_938_RangeSumOfBST.java https://github.com/masikkk/algorithms/blob/master/leetcode/leetcode/_938_RangeSumOfBST.java