二叉樹的LCA問題 ||LeetCode總結：Tree

05-21

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236.Lowest Common Ancestor of a Binary Tree

Lowest Common Ancestor of a Binary Tree - LeetCode?

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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______

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___5__ ___1__
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6 _2 0 8

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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.

class Solution {public: TreeNode* lowestCommonAncestor(TreeNode* root, TreeNode* p, TreeNode* q) { if(root==NULL)return NULL; if(root==p||root==q)return root;//情形一：樹的根節點為要查找的兩點之一，返回根結點 TreeNode* left=lowestCommonAncestor(root->left,p,q);//向左子樹搜索 TreeNode* right=lowestCommonAncestor(root->right,p,q);//向右子樹搜索 if((NULL!=left&&NULL!=right))return root;//情形二：要查找的兩點分別在左右兩子樹，返回根結點 else if(NULL!=left)return left;//情形三：要查找的兩點在同一子樹上，返回先找到的那個結點 else if(NULL!=right)return right; return NULL; }};