530. 二叉搜索树的最小绝对差
题目连接:https://leetcode.cn/problems/minimum-absolute-difference-in-bst/
思路一:使用中序遍历,前后两个之差是最小的绝对差 (递归法)
/**
* 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 {
private TreeNode pre;
private int minV = Integer.MAX_VALUE;
private void getMin(TreeNode root) {
if (root == null) return;
getMin(root.left);
if (pre != null) minV = Math.min(minV, root.val - pre.val);
pre = root;
getMin(root.right);
}
public int getMinimumDifference(TreeNode root) {
getMin(root);
return minV;
}
}
思路二:使用迭代法中序遍历
/**
* 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 {
public int getMinimumDifference(TreeNode root) {
Stack<TreeNode> stack = new Stack<>();
TreeNode cur = root;
TreeNode pre = null;
int minV = Integer.MAX_VALUE;
while (cur != null || !stack.isEmpty()){
if (cur != null) {
stack.push(cur);
cur = cur.left;
} else {
TreeNode treeNode = stack.pop();
if (pre != null) minV = Math.min(minV, treeNode.val - pre.val);
pre = treeNode;
cur = treeNode.right;
}
}
return minV;
}
}
二、 501. 二叉搜索树中的众数
题目连接:https://leetcode.cn/problems/find-mode-in-binary-search-tree/
思路一:使用二叉搜索树中序遍历单调递增有序的特性,使用中序遍历,统计出现的频次,前后相等的count++,前后不相等的或者第一个元素count = 1; if (count == maxCount) 将这个元素加入结合中,如果有比maxcount更大的值,则清楚集合,加入新集合
1、递归法
/**
* 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 {
private TreeNode pre;
private int count = 0;
private int maxCount = 0;
private void findMode(TreeNode root, List<Integer> list) {
if (root == null) return ;
findMode(root.left, list);
if (pre == null) {
count = 1;
} else if (pre.val == root.val) {
count++;
} else {
count = 1;
}
if (count == maxCount) {
list.add(root.val);
} else if (count > maxCount){
list.clear();
list.add(root.val);
maxCount = count;
}
pre = root;
findMode(root.right, list);
}
public int[] findMode(TreeNode root) {
List<Integer> list = new ArrayList<>();
findMode(root, list);
int[] result = new int[list.size()];
for (int i = 0; i < result.length; i++){
result[i] = list.get(i);
}
return result;
}
}
2、使用迭代法中序
/**
* 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 {
public int[] findMode(TreeNode root) {
List<Integer> list = new ArrayList<>();
Stack<TreeNode> stack = new Stack<>();
TreeNode pre = null;
int count = 0;
int maxCount = 0;
TreeNode cur = root;
while (cur != null || !stack.isEmpty()){
if (cur != null) {
stack.push(cur);
cur = cur.left;
} else {
TreeNode treeNode = stack.pop();
if (pre == null || pre.val != treeNode.val) {
count = 1;
} else {
count++;
}
if (count == maxCount) {
list.add(treeNode.val);
} else if (count > maxCount) {
list.clear();
list.add(treeNode.val);
maxCount = count;
}
pre = treeNode;
cur = treeNode.right;
}
}
int[] result = new int[list.size()];
for (int i = 0; i < result.length; i++){
result[i] = list.get(i);
}
return result;
}
}
236. 二叉树的最近公共祖先
题目连接:https://leetcode.cn/problems/lowest-common-ancestor-of-a-binary-tree/
思路:最新公共祖先,使用后序遍历,左边不为空,右边不为空返回中间节点,即使最近公共祖先
/**
* Definition for a binary tree node.
* public class TreeNode {
* int val;
* TreeNode left;
* TreeNode right;
* TreeNode(int x) { val = x; }
* }
*/
class Solution {
public TreeNode lowestCommonAncestor(TreeNode root, TreeNode p, TreeNode q) {
if (root == null) return null;
if (root == p || root == q) return root;
TreeNode leftNode = lowestCommonAncestor(root.left, p, q);
TreeNode rightNode = lowestCommonAncestor(root.right, p, q);
if (leftNode != null && rightNode != null) return root;
else if (leftNode != null && rightNode == null) return leftNode;
else if (leftNode == null && rightNode != null) return rightNode;
else return null;
}
}
网友评论