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  • 【Lowest Common Ancestor of a Binary Tree】cpp

    题目:

    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.

    代码:

    /**
     * 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 || root==p || root==q ) return root;
                TreeNode* l = Solution::lowestCommonAncestor(root->left, p, q);
                TreeNode* r = Solution::lowestCommonAncestor(root->right, p, q);
                if ( l && r )
                {
                    return root;
                }
                else
                {
                    return l ? l : r;
                }   
        }
    };

    tips:

    直接学习大神的思路(http://bookshadow.com/weblog/2015/07/13/leetcode-lowest-common-ancestor-binary-tree/),确实很巧妙。

    总体来说,就是分叉找;终止条件是到了叶子或者找到p和q的一个就返回了。

    这里有个思维误区,为啥找到p或q的一个就返回了,如果先找到p而q在p的下面呢?那就正好了,反正这俩点如果q在p的下面,那么根据定义p就是p和q的公共祖先。

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  • 原文地址:https://www.cnblogs.com/xbf9xbf/p/4644872.html
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