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：递归。如果root为空或者root为p或者q节点，那么直接返回root。递归求左右子树中是否有p和q的最近公共祖先。如果左右子树递归求得的结果均不为空，那么表示p和q分属左右子树，直接返回root，否则p和q均属于左子树或者右子树，那么返回递归求出来的结果。

1. /**
    * Definition for a binary tree node.
    * struct TreeNode {
    *     int val;
    *     TreeNode *left;
    *     TreeNode *right;
    *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
    * };
    */
   class Solution {
   public:
       TreeNode* lowestCommonAncestor(TreeNode* root, TreeNode* p, TreeNode* q) {
           if(!root||p==root||q==root)
               return root;
           TreeNode * left =lowestCommonAncestor(root->left,p,q);
           TreeNode * right=lowestCommonAncestor(root->right,p,q);
           if(left&&right)//p和q分属root两边
               return root;
           return left? left:right;//p和q只是位于左右子树之中的一个
            
       }
   };