另一棵树的子树

题目描述

解法一 DFS

代码演示：

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode() {}
 *     TreeNode(int val) { this.val = val; }
 *     TreeNode(int val, TreeNode left, TreeNode right) {
 *         this.val = val;
 *         this.left = left;
 *         this.right = right;
 *     }
 * }
 */
class Solution {

   	public boolean isSubtree(TreeNode s, TreeNode t) {
        return dfs(s, t);
    }

    public boolean dfs(TreeNode s, TreeNode t){
        if(s == null){
            return false;
        }
        return check(s,t) || dfs(s.left,t) || dfs(s.right,t);
    }

    public boolean check(TreeNode s, TreeNode t){
        if(s == null && t == null){
            return true;
        }
        if(s == null || t == null || s.val != t.val){
            return false;
        }
        return check(s.left,t.left) && check(s.right,t.right);
    }

}

解法二 KMP 算法

代码演示：

	public static boolean isSubtree(TreeNode big, TreeNode small) {
		if (small == null) {
			return true;
		}
		if (big == null) {
			return false;
		}
		ArrayList<String> b = preSerial(big);
		ArrayList<String> s = preSerial(small);
		String[] str = new String[b.size()];
		for (int i = 0; i < str.length; i++) {
			str[i] = b.get(i);
		}

		String[] match = new String[s.size()];
		for (int i = 0; i < match.length; i++) {
			match[i] = s.get(i);
		}
		return getIndexOf(str, match) != -1;
	}

	/**
	 * 前序序列化
	 * @param head
	 * @return
	 */
	public static ArrayList<String> preSerial(TreeNode head) {
		ArrayList<String> ans = new ArrayList<>();
		pres(head, ans);
		return ans;
	}

	/**
	 * 前序遍历
	 * @param head
	 * @param ans
	 */
	public static void pres(TreeNode head, ArrayList<String> ans) {
		if (head == null) {
			ans.add(null);
		} else {
			ans.add(String.valueOf(head.val));
			pres(head.left, ans);
			pres(head.right, ans);
		}
	}

	/**
	 * KMP 算法
	 * @param str1
	 * @param str2
	 * @return
	 */
	public static int getIndexOf(String[] str1, String[] str2) {
		if (str1 == null || str2 == null || str1.length < 1 || str1.length < str2.length) {
			return -1;
		}
		int x = 0;
		int y = 0;
		int[] next = getNextArray(str2);
		while (x < str1.length && y < str2.length) {
			if (isEqual(str1[x], str2[y])) {
				x++;
				y++;
			} else if (next[y] == -1) {
				x++;
			} else {
				y = next[y];
			}
		}
		return y == str2.length ? x - y : -1;
	}

	/**
	 * 前缀数组
	 * @param ms
	 * @return
	 */
	public static int[] getNextArray(String[] ms) {
		if (ms.length == 1) {
			return new int[] { -1 };
		}
		int[] next = new int[ms.length];
		next[0] = -1;
		next[1] = 0;
		int i = 2;
		int cn = 0;
		while (i < next.length) {
			if (isEqual(ms[i - 1], ms[cn])) {
				next[i++] = ++cn;
			} else if (cn > 0) {
				cn = next[cn];
			} else {
				next[i++] = 0;
			}
		}
		return next;
	}

	/**
	 * 判断是否相等
	 * @param a
	 * @param b
	 * @return
	 */
	public static boolean isEqual(String a, String b) {
		if (a == null && b == null) {
			return true;
		} else {
			if (a == null || b == null) {
				return false;
			} else {
				return a.equals(b);
			}
		}
	}