669. 修剪二叉搜索树

力扣题目链接：. - 力扣（LeetCode）

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.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

力扣题目链接：. - 力扣（LeetCode）

循环不变量普遍用于二分查找和递归分割数组

class Solution {public TreeNode sortedArrayToBST(int[] nums) {int length=nums.length;return transform(nums,0,length);}public TreeNode transform(int[] nums,int left,int right){if(right-left<1){return null;}if(right-left==1){return new TreeNode(nums[left]);}int mid=left+(right-left)/2;TreeNode root=new TreeNode(nums[mid]);root.left=transform(nums,left,mid);root.right=transform(nums,mid+1,right);return root;}
}

538.把二叉搜索树转换为累加树

力扣题目链接：. - 力扣（LeetCode）

逆中序遍历

class Solution {int sum;public TreeNode convertBST(TreeNode root) {sum=0;count(root);return root;}public void count(TreeNode root){if(root==null){return;}count(root.right);sum+=root.val;root.val=sum;count(root.left);}
}