Leetcode-235. Lowest Common Ancestor of a Binary Search Tree
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一种复杂的方法:首先将从根节点到p,q两节点的路径上的结点压入栈中,处理两个栈使栈容量保持一致,同时弹出两栈中的元素,直至两栈栈顶元素相同。
class Solution {public: TreeNode* lowestCommonAncestor(TreeNode* root, TreeNode* p, TreeNode* q) { stack<TreeNode*> s1,s2; s1.push(root); do{ if(s1.top()->val < p->val) s1.push(s1.top()->right); else if (s1.top()->val > p->val) s1.push(s1.top()->left); }while(s1.top()!=p); s2.push(root); do{ if(s2.top()->val < q->val) s2.push(s2.top()->right); else if (s2.top()->val > q->val) s2.push(s2.top()->left); } while(s2.top()!=q); while(s1.size()>s2.size()) s1.pop(); while(s2.size()>s1.size()) s2.pop(); while(s1.top() != s2.top()){ s1.pop(); s2.pop(); } return s1.top(); }};
一种非常简单的思路
Just walk down from the whole tree's root as long as both p and q are in the same subtree (meaning their values are both smaller or both larger than root's). This walks straight from the root to the LCA, not looking at the rest of the tree, so it's pretty much as fast as it gets. A few ways to do it:
Iterative, O(1) space
Python
def lowestCommonAncestor(self, root, p, q): while (root.val - p.val) * (root.val - q.val) > 0: root = (root.left, root.right)[p.val > root.val] return root
Java
public TreeNode lowestCommonAncestor(TreeNode root, TreeNode p, TreeNode q) { while ((root.val - p.val) * (root.val - q.val) > 0) root = p.val < root.val ? root.left : root.right; return root;}
(in case of overflow, I'd do (root.val - (long)p.val) * (root.val - (long)q.val)
)
Different Python
def lowestCommonAncestor(self, root, p, q): a, b = sorted([p.val, q.val]) while not a <= root.val <= b: root = (root.left, root.right)[a > root.val] return root
"Long" Python, maybe easiest to understand
def lowestCommonAncestor(self, root, p, q): while root: if p.val < root.val > q.val: root = root.left elif p.val > root.val < q.val: root = root.right else: return root
Recursive
Python
def lowestCommonAncestor(self, root, p, q): next = p.val < root.val > q.val and root.left or \ p.val > root.val < q.val and root.right return self.lowestCommonAncestor(next, p, q) if next else root
Python One-Liner
def lowestCommonAncestor(self, root, p, q): return root if (root.val - p.val) * (root.val - q.val) < 1 else \ self.lowestCommonAncestor((root.left, root.right)[p.val > root.val], p, q)
Java One-Liner
public TreeNode lowestCommonAncestor(TreeNode root, TreeNode p, TreeNode q) { return (root.val - p.val) * (root.val - q.val) < 1 ? root : lowestCommonAncestor(p.val < root.val ? root.left : root.right, p, q);}
"Long" Python, maybe easiest to understand
def lowestCommonAncestor(self, root, p, q): if p.val < root.val > q.val: return self.lowestCommonAncestor(root.left, p, q) if p.val > root.val < q.val: return self.lowestCommonAncestor(root.right, p, q) return root
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