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Nov 5, 2016
Nov 5, 2016

236. Lowest Common Ancestor of a Binary Tree

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.

方法(一)

二种情况。

  1. v和w有直系关系,就像5和6或者6和3
  2. v和w没有直系关系 第二种情况一定分别在T节点的左右子树上,而且也仅有T节点的左右子树分别有这两个节点。这里用递归找左右子树中是否有这两个节点。如果左右子树中都有,那祖先节点就是当前节点,如果只有左或者只有右子树上找到节点,说明是第一种情况返回找到的节点就可以了。