BST find modes

Author: jackzhenguo

Tree/Tree.TreeLib/FindModes.cs

@@ -0,0 +1,83 @@
+using System;
+using System.Collections.Generic;
+using System.Linq;
+using System.Text;
+
+namespace Tree.TreeLib
+{
+//    Given a binary search tree (BST) with duplicates, find all the mode(s) (the most frequently occurred element) in the given BST.
+
+//Assume a BST is defined as follows:
+
+//The left subtree of a node contains only nodes with keys less than or equal to the node's key.
+//The right subtree of a node contains only nodes with keys greater than or equal to the node's key.
+//Both the left and right subtrees must also be binary search trees.
+//For example:
+//Given BST [1,null,2,2],
+//   1
+//    \
+//     2
+//    /
+//   2
+//return [2].
+
+    //求解二叉搜索树中出现次数最多的元素，答案的空间复杂度为 O(1)。
+    //二叉搜索树的中序遍历恰好为元素的从小到大排序，这样相等的元素一定是相邻的。
+    //了解了这个知识点，空间复杂度O(1)就可以做到了。
+    public class Findmodes
+    {
+        private int currentVal;
+        private int currentCount = 0;
+        private int maxCount = 0;
+        private int modeCount = 0;
+        private int[] modeArray;
+
+        public int[] FindMode(TreeNode root)
+        {
+            preorder(root); //第一遍中序遍历找出出现次数最多的元素数，可能有多个最大
+            modeArray = new int[modeCount];
+            modeCount = 0;
+            currentCount = 0;
+            preorder(root);
+            return modeArray;
+        }
+
+        /// <summary>
+        /// 这种方法只适应于二叉搜索树条件下，查找元素出现的最多次数
+        /// </summary>
+        /// <param name="val"></param>
+        private void getModeValue(int val)
+        {
+            if (val != currentVal)
+            {
+                currentVal = val;
+                currentCount = 0;
+            }
+            currentCount++;
+            if (currentCount > maxCount)
+            {
+                maxCount = currentCount;
+                modeCount = 1;
+            }
+            else if (currentCount == maxCount)
+            {
+                if (modeArray != null) //第二遍遍历后，对出现次数最多的元素
+                    modeArray[modeCount] = currentVal; //依次赋值给modeArray
+                modeCount++;
+            }
+        }
+
+        /// <summary>
+        /// 二叉搜索树，采取中序遍历对值处理
+        /// </summary>
+        /// <param name="root"></param>
+        private void preorder(TreeNode root)
+        {
+            if (root == null) 
+                return;
+            preorder(root.left);
+            getModeValue(root.val);
+            preorder(root.right);
+        }
+    }
+}