给你两棵二叉树 root 和 subRoot 。检验 root 中是否包含和 subRoot 具有相同结构和节点值的子树。如果存在,返回 true ;否则,返回 false 。
二叉树 tree 的一棵子树包括 tree 的某个节点和这个节点的所有后代节点。tree 也可以看做它自身的一棵子树。
示例 1:
输入:root = [3,4,5,1,2], subRoot = [4,1,2]
3
/ \
4 5
/ \
1 2
4
/ \
1 2
输出:true
示例 2:
输入:root = [3,4,5,1,2,null,null,null,null,0], subRoot = [4,1,2]
3
/ \
4 5
/ \
1 2
/
0
4
/ \
1 2
输出:false
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结合深度优先递归遍历的算法框架:
当 root 节点的值和 subRoot 节点的值开始相等的时候可以开始判断
boolean isSubtree = false;
public boolean isSubtree(TreeNode root, TreeNode subRoot) {
order(root, subRoot);
return isSubtree;
}
public void order(TreeNode root, TreeNode subRoot) {
if (root == null) {
return;
}
if (root.val == subRoot.val) {
isSubtree = ?
} else {
order(root.left, subRoot);
order(root.right, subRoot);
}
}
当 root 节点的值和 subRoot 节点的值相等时此时可以看成判断两棵树是否相同,可以参考 LeetCode 100. 相同的树:
public boolean isSameTree(TreeNode p, TreeNode q) {
if (p == null && q == null) {
return true;
}
if (p != null && q != null && p.val == q.val) {
return isSameTree(p.left, q.left) && isSameTree(p.right, q.right);
}
return false;
}
完整算法:
boolean isSubtree = false;
public boolean isSubtree(TreeNode root, TreeNode subRoot) {
order(root, subRoot);
return isSubtree;
}
private void order(TreeNode root, TreeNode subRoot) {
if (root == null) {
return;
}
if (root.val == subRoot.val) {
isSubtree = isSameTree(root, subRoot);
} else {
order(root.left, subRoot);
order(root.right, subRoot);
}
}
private boolean isSameTree(TreeNode p, TreeNode q) {
if (p == null && q == null) {
return true;
}
if (p != null && q != null && p.val == q.val) {
return isSameTree(p.left, q.left) && isSameTree(p.right, q.right);
}
return false;
}
提交后,测试用例未通过:
输入:root = [1,1], subRoot = [1]
测试结果:false
期望结果:true
说明思路有问题,只判断一次 root.val == subRoot.val
是不够的,应当需要不断的遍历 root 树,不断比较 root 的子树是否和 subRoot 树一致:
public boolean isSubtree(TreeNode root, TreeNode subRoot) {
boolean rootIsSame = isSameTree(root,subRoot);
boolean leftIsSame = isSubtree(root.left, subRoot);
boolean rightIsSame = isSubtree(root.right, subRoot);
return rootIsSame || leftIsSame || rightIsSame;
}
private boolean isSameTree(TreeNode p, TreeNode q) {
if (p == null && q == null) {
return true;
}
if (p != null && q != null && p.val == q.val) {
return isSameTree(p.left, q.left) && isSameTree(p.right, q.right);
}
return false;
}
补全参数有效性检查
public boolean isSubtree(TreeNode root, TreeNode subRoot) {
if (subRoot == null) {
// subRoot 为 null 一定都是 true
return true;
}
if (root == null) {
// subRoot 一定不为 null, 只要 root 为 null,肯定是 false
return false;
}
boolean rootIsSame = isSameTree(root,subRoot);
boolean leftIsSame = isSubtree(root.left, subRoot);
boolean rightIsSame = isSubtree(root.right, subRoot);
return rootIsSame || leftIsSame || rightIsSame;
}
private boolean isSameTree(TreeNode p, TreeNode q) {
if (p == null && q == null) {
return true;
}
if (p != null && q != null && p.val == q.val) {
return isSameTree(p.left, q.left) && isSameTree(p.right, q.right);
}
return false;
}