Given a binary tree and a sum, find all root-to-leaf paths where each path’s sum equals the given sum.
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## 112. Path Sum

### 二叉树的路径和（Easy）

Given a binary tree and a sum, determine if the tree has a root-to-leaf path such that adding up all the values along the path equals the given sum.
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## 863. All Nodes Distance K in Binary Tree

### Medium

We are given a binary tree (with root node root), a target node, and an integer value K.
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## Top K

References
海量数据处理 - 10亿个数中找出最大的10000个数（top K问题）
549. Top K Frequent Words (Map Reduce)

## Linear Algebra

### 矩阵

特征值和特征向量

## 109. Convert Sorted List to Binary Search Tree

### Medium

Given a singly linked list where elements are sorted in ascending order, convert it to a height balanced BST.
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