Lowest Common Ancestor of a Binary Search Tree

一、題目描寫敘述

二、思路及代碼

二叉搜索樹有個性質：左子樹的值都比根節點小，右子樹的值比根節點大。那麽我們的思路就是遞歸比較。
假設輸入的兩個節點的值比當前節點小，說明是在當前根節點的左子樹中；反之則在右子樹中。

假設當前根節點比一個大。比一個小。那麽第一個出現的這種節點就是近期的父親節點了。

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
 

public class Solution {
    public TreeNode lowestCommonAncestor(TreeNode root, TreeNode p, TreeNode q) {
        if(root.val > p.val && root.val > q.val){
            return lowestCommonAncestor(root.left, p, q);
        }else if(root.val < p.val && root.val < q.val){
            return
 
 lowestCommonAncestor(root.right, p, q);
        }else{
            return root;
        }
    }
}

Lowest Common Ancestor of a Binary Tree

一、題目描寫敘述

二、思路及代碼

假設是普通的二叉樹。沒有了二叉搜索樹的特性。就要遍歷；於是我們用到DFS遍歷樹

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
 

public class Solution {
    public TreeNode lowestCommonAncestor(TreeNode root, TreeNode p, TreeNode q) {
        if(root == null || root == p || root == q) return root; //找到p或者q節點，或者到最底層的葉子節點時，返回；
        TreeNode left = lowestCommonAncestor(root.left, p, q);
        TreeNode right = lowestCommonAncestor(root.right, p, q);

        if(left != null && right != null) return root; //找到了父節點
        return left != null ?
 left : right; //所以假設都未找到，返回null
    }
}

LeetCode_Lowest Common Ancestor of a Binary Search Tree (Binary Tree)