http://www.geeksforgeeks.org/lowest-common-ancestor-binary-tree-set-1/

 #include <iostream>
#include <vector>
#include <algorithm>
#include <queue>
#include <stack>
#include <string>
#include <fstream>
#include <map>
using namespace std; struct node {
int data;
struct node *left, *right;
node() : data(), left(NULL), right(NULL) { }
node(int d) : data(d), left(NULL), right(NULL) { }
}; bool findpath(node *root, vector<int> &path, int n) {
if (!root) return false;
path.push_back(root->data);
if (root->data == n) return true;
if (root->left && findpath(root->left, path, n) || root->right && findpath(root->right, path, n)) return true;
path.pop_back();
return false;
} int LCA(node *root, int n1, int n2) {
if (!root) return -;
vector<int> path1, path2;
if (!findpath(root, path1, n1) || !findpath(root, path2, n2)) return -;
int i = ;
for (; i < path1.size() && i < path2.size(); i++) {
if (path1[i] != path2[i]) break;
}
return path1[i-];
} int main() {
node *root = new node();
root->left = new node();
root->right = new node();
root->left->left = new node();
root->left->right = new node();
root->right->left = new node();
root->right->right = new node();
cout << "LCA(4, 5) is " << LCA(root, , ) << endl;
cout << "LCA(4, 6) is " << LCA(root, , ) << endl;
cout << "LCA(3, 4) is " << LCA(root, , ) << endl;
cout << "LCA(2, 4) is " << LCA(root, , ) << endl;
return ;
}

one traversal

 #include <iostream>
#include <vector>
#include <algorithm>
#include <queue>
#include <stack>
#include <string>
#include <fstream>
#include <map>
using namespace std; struct node {
int data;
struct node *left, *right;
node() : data(), left(NULL), right(NULL) { }
node(int d) : data(d), left(NULL), right(NULL) { }
}; node* LCA(node *root, int n1, int n2) {
if (!root) return NULL;
if (root->data == n1 || root->data == n2) return root;
node *l = LCA(root->left, n1, n2);
node *r = LCA(root->right, n1, n2);
if (l && r) return root;
return l? l : r;
} int main() {
node *root = new node();
root->left = new node();
root->right = new node();
root->left->left = new node();
root->left->right = new node();
root->right->left = new node();
root->right->right = new node();
cout << "LCA(4, 5) is " << LCA(root, , )->data << endl;
cout << "LCA(4, 6) is " << LCA(root, , )->data << endl;
cout << "LCA(3, 4) is " << LCA(root, , )->data << endl;
cout << "LCA(2, 4) is " << LCA(root, , )->data << endl;
return ;
}

上面这段代码有些问题,当n1或者n2不存在时应该回NULL,但是回的是n1或者n2

 #include <iostream>
#include <vector>
#include <algorithm>
#include <queue>
#include <stack>
#include <string>
#include <fstream>
#include <map>
using namespace std; struct node {
int data;
struct node *left, *right;
node() : data(), left(NULL), right(NULL) { }
node(int d) : data(d), left(NULL), right(NULL) { }
}; node* _LCA(node *root, int n1, int n2, bool &v1, bool &v2) {
if (!root) return NULL;
if (root->data == n1) {
v1 = true;
return root;
}
if (root->data == n2) {
v2 = true;
return root;
}
node *l = _LCA(root->left, n1, n2, v1, v2);
node *r = _LCA(root->right, n1, n2, v1, v2);
if (l && r) return root;
return l? l : r;
} bool findnode(node *root, int k) {
if (!root) return false;
return (root->data == k || findnode(root->left, k) || findnode(root->right, k));
} node *LCA(node *root, int n1, int n2) {
bool v1 = false;
bool v2 = false;
node *lca = _LCA(root, n1, n2, v1, v2);
if (v1 && v2 || v1 && findnode(root, n2) || v2 && findnode(root, n1)) return lca;
return NULL;
} int main() {
node *root = new node();
root->left = new node();
root->right = new node();
root->left->left = new node();
root->left->right = new node();
root->right->left = new node();
root->right->right = new node();
cout << "LCA(4, 5) is " << LCA(root, , )->data << endl;
cout << "LCA(4, 6) is " << LCA(root, , )->data << endl;
cout << "LCA(3, 4) is " << LCA(root, , )->data << endl;
cout << "LCA(2, 4) is " << LCA(root, , )->data << endl;
return ;
}

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