You are given a binary tree in which each node contains an integer value.
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257. Binary Tree Paths
Easy
Given a binary tree, return all root-to-leaf paths.
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113. Path Sum II
Medium
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)