LeetCode(235) Lowest Common Ancestor of a Binary Search Tree解题报告

Given a binary search tree (BST), find the lowest common ancestor (LCA) of two given nodes in the BST.

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).”

____6______
/           \
___2__       ___8__
/      \     /      \
0      _4   7        9
/  \
3   5

For example, the lowest common ancestor (LCA) of nodes 2 and 8 is 6. Another example is LCA of nodes 2 and 4 is 2, since a node can be a descendant of itself according to the LCA definition.

解题思路:

因为是BST,所以很好判断，如果root比p,q任何一个都小，那么LCA一定在右子树上，递归，反之，在左子树，如果正好比一个大，比一个小，那么LCA就是root

下面分别附上递归和迭代解法：

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

}

public class Solution {
public TreeNode lowestCommonAncestor(TreeNode root, TreeNode p, TreeNode q) {
while(true){
while(root.val < Math.min(p.val,q.val)){
root = root.right;
}
while(root.val > Math.max(p.val,q.val)){
root = root.left;
}
while(root.val <= Math.max(p.val, q.val) && root.val >= Math.min(p.val,q.val))
return root;
}
}

}