题解 | 树的子结构

import java.util.*;
/**
public class TreeNode {
    int val = 0;
    TreeNode left = null;
    TreeNode right = null;

    public TreeNode(int val) {
        this.val = val;
    }
}
*/

/**
 * NC273 树的子结构
 * @author d3y1
 */
public class Solution {
    private boolean isSubtree = false;

    public boolean HasSubtree(TreeNode root1, TreeNode root2){
        return solution1(root1, root2);
        // return solution2(root1, root2);
    }

    /**
     * 递归: 前序遍历
     * @param root1
     * @param root2
     * @return
     */
    private boolean solution1(TreeNode root1, TreeNode root2){
        if(root1==null || root2 == null){
            return false;
        }

        preOrder(root1, root2);

        return isSubtree;
    }

    /**
     * 前序遍历
     * @param root1
     * @param root2
     */
    private void preOrder(TreeNode root1, TreeNode root2){
        if(isSubtree){
            return;
        }
        if(root1 == null){
            return;
        }

        if(root1.val == root2.val){
            isSubtree = verify(root1, root2);
        }
        preOrder(root1.left, root2);
        preOrder(root1.right, root2);
    }

    /**
     * 校验: 递归
     * @param root1
     * @param root2
     * @return
     */
    private boolean verify(TreeNode root1, TreeNode root2){
        if(root2 == null){
            return true;
        }

        if(root1==null || root1.val!=root2.val){
            return false;
        }else{
            return verify(root1.left, root2.left) && verify(root1.right, root2.right);
        }
    }

    /**
     * 迭代: 层序遍历
     * @param root1
     * @param root2
     * @return
     */
    private boolean solution2(TreeNode root1, TreeNode root2){
        if(root1==null || root2 == null){
            return false;
        }

        levelOrder(root1, root2);

        return isSubtree;
    }

    /**
     * 层序遍历
     * @param root1
     * @param root2
     */
    private void levelOrder(TreeNode root1, TreeNode root2){
        Queue<TreeNode> queue1 = new LinkedList<>();
        queue1.offer(root1);

        TreeNode node1;
        while(!queue1.isEmpty()){
            node1 = queue1.poll();
            if(node1.val == root2.val){
                isSubtree = check(node1, root2);
                if(isSubtree){
                    return;
                }
            }
            if(node1.left != null){
                queue1.offer(node1.left);
            }
            if(node1.right != null){
                queue1.offer(node1.right);
            }
        }
    }

    /**
     * 校验: 队列
     * @param root1
     * @param root2
     * @return
     */
    private boolean check(TreeNode root1, TreeNode root2){
        Queue<TreeNode> queue1 = new LinkedList<>();
        Queue<TreeNode> queue2 = new LinkedList<>();
        queue1.offer(root1);
        queue2.offer(root2);

        TreeNode node1,node2;
        while(!queue2.isEmpty()){
            node1 = queue1.poll();
            node2 = queue2.poll();
            if(node1==null || node1.val!=node2.val){
                return false;
            }
            if(node2.left != null){
                queue1.offer(node1.left);
                queue2.offer(node2.left);
            }
            if(node2.right != null){
                queue1.offer(node1.right);
                queue2.offer(node2.right);
            }
        }

        return true;
    }
}