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

    1. 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 p and q as the lowest node in T that has both p and q as descendants (where we allow a node to be a descendant of itself).”

    Given the following binary tree: root = [3,5,1,6,2,0,8,null,null,7,4]

    img

    Example 1:

    Input: root = [3,5,1,6,2,0,8,null,null,7,4], p = 5, q = 1
    Output: 3
    Explanation: The LCA of nodes 5 and 1 is 3.
    

    Example 2:

    Input: root = [3,5,1,6,2,0,8,null,null,7,4], p = 5, q = 4
    Output: 5
    Explanation: The LCA of nodes 5 and 4 is 5, since a node can be a descendant of itself according to the LCA definition.
    

    Note:

    • All of the nodes' values will be unique.
    • p and q are different and both values will exist in the binary tree.

    对于root节点,如果左右子树分别有一个节点,则root是一个公共祖先

    解1 递归

    class Solution {
    public:
        TreeNode *ans;
        TreeNode* lowestCommonAncestor(TreeNode* root, TreeNode* p, TreeNode* q) {
            LCA(root, p, q);
            return ans;
        }
        bool LCA(TreeNode *root, TreeNode *p, TreeNode *q){
            if(root == NULL)return false;
            int l = LCA(root->left, p, q) ? 1 : 0; // p或者q在左子树
            int r = LCA(root->right, p, q) ? 1 : 0; // p或者q在右子树
            int mid = (root == p || root == q) ? 1 : 0; //找到了p或者q
            if(mid + l + r >= 2)ans = root; // =2时,左右各一个;>2时,一个是另一个的先驱节点
            return mid + l + r > 0; // >0表明找到了p或者q
        }
    };
    
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  • 原文地址:https://www.cnblogs.com/vinnson/p/13355820.html
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