feat: create 501. 二叉搜索树中的众数.md

501. 二叉搜索树中的众数.md

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+
+# [501. 二叉搜索树中的众数](https://leetcode.cn/problems/find-mode-in-binary-search-tree/)
+
+- 队列 + 哈希表
+
+```
+/**
+ * Definition for a binary tree node.
+ * class TreeNode {
+ *     val: number
+ *     left: TreeNode | null
+ *     right: TreeNode | null
+ *     constructor(val?: number, left?: TreeNode | null, right?: TreeNode | null) {
+ *         this.val = (val===undefined ? 0 : val)
+ *         this.left = (left===undefined ? null : left)
+ *         this.right = (right===undefined ? null : right)
+ *     }
+ * }
+ */
+
+function findMode(root: TreeNode | null): number[] {
+    const queue: TreeNode[] = [root];
+    const frequencyMap: Map<number, number> = new Map<number, number>();
+    let maxFrequency: number = 0;
+
+    while(queue.length) {
+        // 访问节点
+        const current = queue.pop();
+        // 记录频率
+        frequencyMap.set(current.val, (frequencyMap.get(current.val) ?? 0) + 1);
+        // 统计最大值
+        maxFrequency = Math.max(maxFrequency, frequencyMap.get(current.val));
+
+        current.left && queue.push(current.left);
+        current.right && queue.push(current.right);
+    }
+
+    return [...frequencyMap.keys()].reduce((results, key) => {
+        if(frequencyMap.get(key) === maxFrequency) {
+            results.push(key);
+        }
+        return results;
+    }, []) 
+};
+```