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  • 【LeetCode】236. Lowest Common Ancestor of a Binary Tree

    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.

    好巧,我在Lowest Common Ancestor of a Binary Search Tree的解法一,

    就是这题的解法。

    深度遍历到节点p时,栈中的所有节点即为p的从根开始的祖先序列。

    因此只需要比较p、q祖先序列中最后一个相同的祖先即可。

    /**
     * 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) {
            // special cases
            if(root == NULL)
                return NULL;
            if(p == root || q == root)
                return root;
            if(p == q)
                return p;
                
            vector<TreeNode*> vp;
            vector<TreeNode*> vq;
            stack<TreeNode*> stk;
            unordered_map<TreeNode*, bool> m;   //visited
            stk.push(root);
            m[root] = true;
            while(!stk.empty())
            {
                TreeNode* top = stk.top();
                if(top->left && m[top->left] == false)
                {
                    stk.push(top->left);
                    m[top->left] = true;
                    if(top->left == p)
                    {
                        vp = stkTovec(stk);
                        if(!vq.empty())
                            break;
                    }
                    if(top->left == q)
                    {
                        vq = stkTovec(stk);
                        if(!vp.empty())
                            break;
                    }
                    continue;
                }
                if(top->right && m[top->right] == false)
                {
                    stk.push(top->right);
                    m[top->right] = true;
                    if(top->right == p)
                    {
                        vp = stkTovec(stk);
                        if(!vq.empty())
                            break;
                    }
                    if(top->right == q)
                    {
                        vq = stkTovec(stk);
                        if(!vp.empty())
                            break;
                    }
                    continue;
                }
                stk.pop();
            }
            int i = 0;
            for(; i < vp.size() && i < vq.size(); i ++)
            {
                if(vp[i] != vq[i])
                    break;
            }
            return vp[i-1];
        }
        vector<TreeNode*> stkTovec(stack<TreeNode*> stk)
        {
            vector<TreeNode*> v;
            while(!stk.empty())
            {
                TreeNode* top = stk.top();
                stk.pop();
                v.push_back(top);
            }
            reverse(v.begin(), v.end());
            return v;
        }
    };

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  • 原文地址:https://www.cnblogs.com/ganganloveu/p/4642083.html
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