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  • Leetcode: 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.

    Recursion

     1 /**
     2  * Definition for a binary tree node.
     3  * public class TreeNode {
     4  *     int val;
     5  *     TreeNode left;
     6  *     TreeNode right;
     7  *     TreeNode(int x) { val = x; }
     8  * }
     9  */
    10 public class Solution {
    11     public TreeNode lowestCommonAncestor(TreeNode root, TreeNode p, TreeNode q) {
    12         if (root==null || p==null || q==null) return null;
    13         if (root==p || root==q) return root;
    14         TreeNode left = lowestCommonAncestor(root.left, p, q);
    15         TreeNode right = lowestCommonAncestor(root.right, p, q);
    16         if (left!=null && right==null) return left;
    17         if (left==null && right!=null) return right;
    18         if (left!=null && right!=null) return root; //found LCA
    19         else return null;
    20     }
    21 }
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  • 原文地址:https://www.cnblogs.com/EdwardLiu/p/5060241.html
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