[LeetCode] 236. Lowest Common Ancestor of a Binary Tree

轉自LeetCode

Given a binary tree, find the lowest common ancestor (LCA) of two given nodes in the tree.

According to the definition of LCA on Wikipedia: “The lowest common ancestor is defined between two nodes v and w as the lowest node in T that has both v and w as descendants (where we allow a node to be a descendant of itself).”

        _______3______
       /              \
    ___5__          ___1__
   /      \        /      \
   6      _2       0       8
         /  \
         7   4

For example, the lowest common ancestor (LCA) of nodes 5 and 1 is 3. Another example is LCA of nodes 5 and 4 is 5, since a node can be a descendant of itself according to the LCA definition.

<Solution>

這題是 235. Lowest Common Ancestor of a Binary Search Tree  的衍生題

差別在於，這次的 data structure 不是 BST 而是一般的 binary tree 而已

那想法也是使用 recursive 來解(DFS)

想法如下

• 如果能分別在左右兩個子樹，各自找到 v 和 w，那麼當下的 root 就是 LCA

• 如果分別找左右兩個子樹時，只能找到 v 或 w，也就是只能找到一個，那麼就代表 v 和 w 在同一邊，且找到的那個節點就是 LCA

code 如下

Java

kotlin