LeetCode 236. Lowest Common Ancestor of a Binary Tree--递归
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题目链接
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.
1.root是NULL返回NULL
2.root是p or q,返回root
3.root->left返回值不是NULL && root->right返回值不是NULL,说明root即为所求,返回root
4.root->left返回值不是NULL or root->right返回值不是NULL,说明不为NULL的返回值即为所求或为p,q,返回它
5.没找到p or q,返回NULL
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 == NULL || root == p || root == q) return root; TreeNode* l = lowestCommonAncestor(root->left, p, q); TreeNode* r = lowestCommonAncestor(root->right, p, q); if (l && r) return root; if (l) return l; if (r) return r; return NULL; }};
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