236. Lowest Common Ancestor of a Binary Tree

Posted 2020-08-09 Machelsky

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.

思路：搜索左右子树，如果左子树有lca，右子树没有。lca在左侧。左子树没有，右子树有，在右侧。两侧都有，则在root。

ref:https://www.youtube.com/watch?v=13m9ZCB8gjw

这小哥讲的挺好。

/**
 * 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)
        {
            return root;
        }
         if( root == p || root == q )
            return root;
        TreeNode l1=lowestCommonAncestor(root.left,p,q);
        TreeNode l2=lowestCommonAncestor(root.right,p,q);
        if(l1!=null&&l2!=null)
        {
            return root;
        }
        if(l1!=null)
        {
            return l1;
        }
        else
        {
            return l2;
        }
    }
}