Leetcode 236: Lowest Common Ancestor of a Binary Tree, leetcodeancestor

Leetcode 236: Lowest Common Ancestor of a Binary Tree, leetcodeancestor
Lowest Common Ancestor of a Binary TreeTotal Accepted:1628Total Submissions:5660

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 allowA node to be a descendant of itself)."

        _______3______       /              \    ___5__          ___1__   /      \        /      \   6      _2       0       8         /  \         7   4

For example, the lowest common ancestor (LCA) of nodes5And1Is3. Another example is LCA of nodes5And4Is5, Since a node can be a descendant of itself according to the LCA definition.

[Idea]

If it is a common Binary Tree, rather than a BST. then we should traverse the node and find p, q. at the same time, record the path from root to this point. then compare the path, and the last same node is LCA.

[CODE]

/** * Definition for a binary tree node. * public class TreeNode { *     int val; *     TreeNode left; *     TreeNode right; *     TreeNode(int x) { val = x; } * } */public class Solution {    //2, 1    public TreeNode lowestCommonAncestor(TreeNode root, TreeNode p, TreeNode q) {        if(root==null || p==null || q==null) return null;                List<TreeNode> pathp = new ArrayList<>();        List<TreeNode> pathq = new ArrayList<>();        pathp.add(root);        pathq.add(root);                getPath(root, p, pathp);        getPath(root, q, pathq);                TreeNode lca = null;        for(int i=0; i<pathp.size() && i<pathq.size(); i++) {            if(pathp.get(i) == pathq.get(i)) lca = pathp.get(i);            else break;        }        return lca;    }        private boolean getPath(TreeNode root, TreeNode n, List<TreeNode> path) {        if(root==n) {            return true;        }                if(root.left!=null) {            path.add(root.left);            if(getPath(root.left, n, path)) return true;            path.remove(path.size()-1);        }                if(root.right!=null) {            path.add(root.right);            if(getPath(root.right, n, path)) return true;            path.remove(path.size()-1);        }                return false;    }}

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