Article directory
1. Search in binary search tree
Topic link: 700. Search in a binary search tree
/**
* <pre>
* 1.递归
* 2.迭代
* </pre>
*
* @author <a href="https://github.com/Ken-Chy129">Ken-Chy129</a>
* @date 2023/1/17 14:33
*/
public class 二叉搜索树中的搜索700 {
public static TreeNode searchBST(TreeNode root, int val) {
if (root == null || root.val == val) {
return root;
}
if (val < root.val) {
return searchBST(root.left, val);
} else {
return searchBST(root.right, val);
}
}
public static TreeNode searchBST2(TreeNode root, int val) {
while (root != null) {
if (root.val == val) {
return root;
} else if (val < root.val) {
root = root.left;
} else {
root = root.right;
}
}
return null;
}
}
Second, verify the binary search tree
Topic link: 98. Verify binary search tree
/**
* <pre>
* 1.递归，每次判断根据上下限判断当前节点是否满足条件
* 2.中序遍历：二叉搜索树中序遍历后的顺序为从小到大
* </pre>
*
* @author <a href="https://github.com/Ken-Chy129">Ken-Chy129</a>
* @date 2023/1/17 16:21
*/
public class 验证二叉搜索树98 {
public boolean isValidBST(TreeNode root) {
return isValidBST(root, Long.MIN_VALUE, Long.MAX_VALUE);
}
public boolean isValidBST(TreeNode node, long lower, long upper) {
if (node == null) {
return true;
}
if (node.val >= upper || node.val <= lower) {
return false;
}
return isValidBST(node.left, lower, node.val) && isValidBST(node.right, node.val, upper);
}
public boolean isValidBST2(TreeNode root) {
Stack<TreeNode> stack = new Stack<>();
long inorder = Long.MIN_VALUE;
while (!stack.empty() || root != null) {
while (root != null) {
stack.push(root);
root = root.left;
}
TreeNode pop = stack.pop();
if (pop.val <= inorder) {
// 二叉搜索树中序遍历的节点应该越来越大
return false;
}
inorder = pop.val;
root = pop.right;
}
return true;
}
}
3. The minimum absolute difference of the binary search tree
Topic link: 530. Minimum absolute difference of binary search tree
/**
* <pre>
* 1.中序遍历，出来的结果是升序排序，插值最小肯定存在在两个相邻元素之间
* </pre>
*
* @author <a href="https://github.com/Ken-Chy129">Ken-Chy129</a>
* @date 2023/1/17 16:58
*/
public class 二叉搜索树的最小绝对差530 {
public int getMinimumDifference(TreeNode root) {
Stack<TreeNode> stack = new Stack<>();
long last = Integer.MIN_VALUE, ans = Integer.MAX_VALUE;
while (!stack.empty() || root != null) {
while (root != null) {
stack.push(root);
root = root.left;
}
TreeNode pop = stack.pop();
ans = Math.min(ans, pop.val - last);
last = pop.val;
root = pop.right;
}
return (int)ans;
}
}
Four, the mode in the binary search tree
Topic Link: 501. Modes in Binary Search Trees
/**
* <pre>
* 1.中序遍历
* </pre>
*
* @author <a href="https://github.com/Ken-Chy129">Ken-Chy129</a>
* @date 2023/1/17 17:29
*/
public class 二叉搜索树中的众数501 {
public int[] findMode(TreeNode root) {
Stack<TreeNode> stack = new Stack<>();
List<Integer> list = new ArrayList<>();
int maxCount = 0, count = 0, last = Integer.MIN_VALUE;
while (!stack.empty() || root != null) {
while (root != null) {
stack.push(root);
root = root.left;
}
TreeNode pop = stack.pop();
if (last == pop.val) {
count++;
} else {
count = 1;
}
if (maxCount == count) {
list.add(pop.val);
} else if (maxCount < count) {
list.clear();
list.add(pop.val);
maxCount = count;
}
last = pop.val;
root = pop.right;
}
int[] res = new int[list.size()];
for (int i=0; i<list.size(); i++) {
res[i] = list.get(i);
}
return res;
}
}