Subtree

You have two every large binary trees: T1, with millions of nodes, and T2, with hundreds of nodes. Create an algorithm to decide if T2 is a subtree ofT1.

A tree T2 is a subtree of T1 if there exists a node n in T1 such that the subtree of n is identical to T2. That is, if you cut off the tree at node n, the two trees would be identical.

思路：

　　　　1.判断当前结点是否是子树的根节点（判断以当前节点构成的树是否与子树相同）， 判断当前的左孩子是否为子树的根节点，判断当前的右孩子是否为子树的根节点。

　　　　　　

/**
 * Definition of TreeNode:
 * public class TreeNode {
 *     public int val;
 *     public TreeNode left, right;
 *     public TreeNode(int val) {
 *         this.val = val;
 *         this.left = this.right = null;
 *     }
 * }
 */
public class Solution {
    /**
     * @param T1, T2: The roots of binary tree.
     * @return: True if T2 is a subtree of T1, or false.
     */
    public boolean isSubtree(TreeNode T1, TreeNode T2) {
       
        if (T1 == null) {
            return T1 == T2;
        }
        if (T2 == null) {
            return true;
        }
        if (isEqual(T1, T2)){
            return true;
        }
        if (isSubtree(T1.left, T2)) {
            return true;
        }
        if (isSubtree(T1.right, T2)) {
            return true;
        }
        return false;
        
    }

    private boolean isEqual(TreeNode root1, TreeNode root2) {
        if (root1 == null && root2 == null) {
            return true;
        }
        if (root1 == null || root2 == null) {
            return root1 == root2;
        }
        if (root1.val != root2.val) {
            return false;
        }
        return isEqual(root1.left, root2.left) && isEqual(root1.right, root2.right);
    }
}