Medium 236题 Lowest Common Ancestor of a Binary Tree

Question:

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:

/** * Definition for a binary tree node. * public class TreeNode { *     int val; *     TreeNode left; *     TreeNode right; *     TreeNode(int x) { val = x; } * } */public class Solution {    public TreeNode lowestCommonAncestor(TreeNode root, TreeNode p, TreeNode q) {        if(root==null||root==p||root==q)            return root;        TreeNode left=lowestCommonAncestor(root.left,p,q);        TreeNode right=lowestCommonAncestor(root.right,p,q);                if(left!=null)        {            if(right!=null)                return root;            else                return left;        }                if(right!=null)        {            if(left!=null)                return root;            else                return right;        }        return null;    }}