236. Lowest Common Ancestor of a Binary Tree java solutions

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 allow a node to be a descendant of itself).”

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

For example, the lowest common ancestor (LCA) of nodes 5 and 1 is 3. Another example is LCA of nodes 5 and 4 is 5, since a node can be a descendant of itself according to the LCA definition.

 

建立节点的parent 字典，p 、q 分别往前遍历即可。

/**
 * 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) {
        Map<TreeNode,TreeNode> parent = new HashMap<TreeNode,TreeNode>();
        Deque<TreeNode> s = new ArrayDeque<TreeNode>();
        parent.put(root,null);
        s.add(root);
        while(!parent.containsKey(p) || !parent.containsKey(q)){
            TreeNode n = s.pop();
            if(n.left != null){
                parent.put(n.left,n);
                s.add(n.left);
            }
            if(n.right != null){
                parent.put(n.right,n);
                s.add(n.right);
            }
        }
        Set<TreeNode> set = new HashSet<TreeNode>();
        while(p!= null){
            set.add(p);
            p = parent.get(p);
        }
        while(!set.contains(q)) q = parent.get(q);
        return q;
    }
}