法一：下标法
大的下标除以2，直至两数下标相同，即为公共前驱下标。
测试用例里面会有一个超长的集合，下标会超出浮点的表示范围，导致出错，只能另寻它法
/**
* 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:
float getIndex(TreeNode* root, float index, int value){
if(root == NULL) return -1;
if(value == root->val) {
cout<< index << endl;
return index;
}
float returnValue = getIndex(root->left, 2*index, value);
if(returnValue > 0)return returnValue;
returnValue = getIndex(root->right, 2*index+1, value);
if(returnValue > 0)return returnValue;
return -1;
}
TreeNode* getNode(TreeNode* root, float index, float targetIndex, int &statute){
if(root == NULL) return NULL;
//cout<<"value="<<root->val <<" index="<< index<<endl;
if(index == targetIndex){
statute = 1;
return root;
}
TreeNode* tmpNode = getNode(root->left, 2*index, targetIndex, statute);
if(statute == 1)return tmpNode;
tmpNode = getNode(root->right, 2*index+1, targetIndex, statute);
if(statute == 1)return tmpNode;
return NULL;
}
TreeNode* lowestCommonAncestor(TreeNode* root, TreeNode* p, TreeNode* q) {
float index1 = getIndex(root, 1, p->val);
float index2 = getIndex(root, 1, q->val);
while(index1>1 && index2>1){
if(index1 == index2) break;
if(index1 > index2) index1 = floor(index1/2);
else index2 = floor(index2/2);
}
cout<<"index1="<< index1 << " index2="<<index2<<endl;
if(index1 == 1 || index2 == 1) return root;
else{
int statute = 0;
return getNode(root, 1, index1, statute);
}
}
};
法二：利用堆栈
/**
* 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:
//将根节点到该点的路径push到栈中
void getPath(vector<TreeNode*>& path, int& statute,TreeNode* p, int targetValue){
if(statute == 1 || p == NULL) return;
if(p->val == targetValue) {
//cout<<" push "<<p->val;
path.push_back(p);
statute = 1;
return;
}
path.push_back(p);
getPath(path, statute, p->left, targetValue);
if(statute == 1) return;
getPath(path, statute, p->right, targetValue);
if(statute == 1) return;
//未找到目标节点，回溯
path.pop_back();
return;
}
TreeNode* lowestCommonAncestor(TreeNode* root, TreeNode* p, TreeNode* q) {
vector<TreeNode*> path_p;
vector<TreeNode*> path_q;
//标志是否找到该元素
int statute = -1;
getPath(path_p, statute, root, p->val);
statute = -1;
getPath(path_q, statute, root, q->val);
while(path_p.size()>1 && path_q.size()>1){
int pPathLength = path_p.size();
int qPathLength = path_q.size();
//cout<<endl<<" plength="<<pPathLength<<" qlength="<<qPathLength<<endl;
if(path_p[pPathLength-1]->val == path_q[qPathLength - 1]->val) {
//cout<<"ptop="<<path_p[pPathLength-1]->val<<" qtop="<<path_q[qPathLength - 1]->val;
break;
}
//如果找到公共路径，两栈的长度一定是一样长的
//将长度较长的栈pop栈顶元素
if (pPathLength >= qPathLength) {
path_p.pop_back();
pPathLength--;
}
else if (pPathLength < qPathLength) {
path_q.pop_back();
qPathLength--;
}
}
if(path_p.size() == 1 || path_q.size() == 1) return root;
else return path_p[path_p.size() - 1];
}
};
