Leetcode236二叉樹的最近公共祖先

1.遞歸
2 迭代
棧存儲遍歷結點
HashMap 存儲祖先結點
HashSet 存儲p的祖先結點
找p在q祖先結點中存在的第一個結點；

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
class Solution {
    private TreeNode ans;
    public Solution(){
        this.ans = null;
    }
    private boolean recurseTree(TreeNode curr,TreeNode p,TreeNode q){
        if(curr == null){return false;}
        //set left
        int left = recurseTree(curr.left,p,q) ? 1:0;
        int right = recurseTree(curr.right,p,q) ? 1:0;
        int mid = (curr == p || curr == q) ? 1:0;
        if(left + mid + right >= 2){
            this.ans = curr;
        }
        return (left + mid + right > 0);
    }
    public TreeNode lowestCommonAncestor(TreeNode root, TreeNode p, TreeNode q) {
        this.recurseTree(root,p,q);
        return this.ans;
    }




    //解法2 迭代 棧
        public TreeNode lowestCommonAncestor2(TreeNode root, TreeNode p, TreeNode q) {
       //stack for traversal
        Deque<TreeNode> stack = new ArrayDeque<>();
        //map for parent pointers
        Map<TreeNode,TreeNode> parent = new HashMap<>();
        stack.push(root);
        parent.put(root,null);
        while(!parent.containsKey(p) || !parent.containsKey(q)){
            TreeNode node = stack.pop();
            //將結點的左右子樹的根加入到parent中
            if(node.left != null){
                parent.put(node.left,node);
                stack.push(node.left);
            }
            if(node.right != null){
                parent.put(node.right,node);
                stack.push(node.right);
            }
        }
        //存儲p結點的祖先
        Set<TreeNode> ancestors = new HashSet<>();
        while(p!=null){
            ancestors.add(p);
            p= parent.get(p);
        }
        //p的ancestors裏面找和q一樣的公共祖先
        while(!ancestors.contains(q)){
            q = parent.get(q);
        }
        return q;
    }
}