Lowest Common Ancestor of a Binary Tree
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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.
方法一:根据路径找LCA。
1 /** 2 * Definition for a binary tree node. 3 * struct TreeNode { 4 * int val; 5 * TreeNode *left; 6 * TreeNode *right; 7 * TreeNode(int x) : val(x), left(NULL), right(NULL) {} 8 * }; 9 */10 class Solution {11 public:12 void getPath(TreeNode *root, TreeNode *p, TreeNode *q, vector<TreeNode*> &path, vector<TreeNode *> &path1, vector<TreeNode*> &path2) {13 if (root == NULL) return;14 path.push_back(root);15 if (root == p) path1 = path;16 if (root == q) path2 = path;17 //找到两个节点后就可以退出了18 if (!path1.empty() && !path2.empty()) return;19 getPath(root->left, p, q, path, path1, path2);20 getPath(root->right, p, q, path, path1, path2);21 path.pop_back();22 }23 24 TreeNode* lowestCommonAncestor(TreeNode* root, TreeNode* p, TreeNode* q) {25 vector<TreeNode*> path, path1, path2;26 getPath(root, p, q, path, path1, path2);27 TreeNode *res = root;28 int idx = 0;29 while (idx < path1.size() && idx < path2.size()) {30 if (path1[idx] != path2[idx]) break;31 else res = path1[idx++];32 }33 return res;34 }35 };
方法二:节点a与节点b的公共祖先c一定满足:a与b分别出现在c的左右子树上(如果a或者b本身不是祖先的话)。
1 /** 2 * Definition for a binary tree node. 3 * struct TreeNode { 4 * int val; 5 * TreeNode *left; 6 * TreeNode *right; 7 * TreeNode(int x) : val(x), left(NULL), right(NULL) {} 8 * }; 9 */10 class Solution {11 public:12 TreeNode* lowestCommonAncestor(TreeNode* root, TreeNode* p, TreeNode* q) {13 if (root == NULL) return NULL;14 if (root == p || root == q) return root;15 TreeNode *L = lowestCommonAncestor(root->left, p, q);16 TreeNode *R = lowestCommonAncestor(root->right, p, q);17 if (L && R) return root;18 return L ? L : R;19 }20 };
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- Lowest Common Ancestor of a Binary Tree
- Lowest Common Ancestor of a Binary Tree
- Lowest Common Ancestor of a Binary Tree
- Lowest Common Ancestor of a Binary Tree
- Lowest Common Ancestor of a Binary Tree
- Lowest Common Ancestor of a Binary Tree
- Lowest Common Ancestor of a Binary Tree
- Lowest Common Ancestor of a Binary Tree
- Lowest Common Ancestor of a Binary Tree
- Lowest Common Ancestor of a Binary Tree(***)
- Lowest Common Ancestor of a Binary Tree
- Lowest Common Ancestor of a Binary Tree
- Lowest Common Ancestor of a Binary Tree
- Lowest Common Ancestor of a Binary Tree
- Lowest Common Ancestor of a Binary Tree
- Lowest Common Ancestor of a Binary Tree
- Lowest Common Ancestor of a Binary Tree
- Lowest Common Ancestor of a Binary Tree
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