代码随想录训练营Day21

669. 修剪二叉搜索树 leetcode.com/problems/tr…

思路：主要还是递归的能力

class Solution {
    public TreeNode trimBST(TreeNode root, int low, int high) {
        if (root == null) {
            return null;
        }
        if (root.val < low) {
            return trimBST(root.right, low, high);
        }
        if (root.val > high) {
            return trimBST(root.left, low, high);
        }
        // root在[low,high]范围内
        root.left = trimBST(root.left, low, high);
        root.right = trimBST(root.right, low, high);
        return root;
    }
}

108.将有序数组转换为二叉搜索树 leetcode.com/problems/co…

class Solution {
	public TreeNode sortedArrayToBST(int[] nums) {
		TreeNode root = traversal(nums, 0, nums.length - 1);
		return root;
	}

	// 左闭右闭区间[left, right]
	private TreeNode traversal(int[] nums, int left, int right) {
		if (left > right) return null;

		int mid = left + ((right - left) >> 1);
		TreeNode root = new TreeNode(nums[mid]);
		root.left = traversal(nums, left, mid - 1);
		root.right = traversal(nums, mid + 1, right);
		return root;
	}
}

538.把二叉搜索树转换为累加树 leetcode.com/problems/co…

思路：这个比较简单就是递归遍历就行

class Solution {
    int sum;
    public TreeNode convertBST(TreeNode root) {
        sum = 0;
        convertBST1(root);
        return root;
    }

    // 按右中左顺序遍历，累加即可
    public void convertBST1(TreeNode root) {
        if (root == null) {
            return;
        }
        convertBST1(root.right);
        sum += root.val;
        root.val = sum;
        convertBST1(root.left);
    }
}