剑指Offer之Java算法习题精讲二叉树专项解析
作者：明天一定.
跟着思路走，之后从简单题入手，反复去看，做过之后可能会忘记，之后再做一次，记不住就反复做，反复寻求思路和规律，慢慢积累就会发现质的变化
题目一
解法
/**
* 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 {
int ans;
int pre;
public int getMinimumDifference(TreeNode root) {
ans = Integer.MAX_VALUE;
pre = -1;
method(root);
return ans;
}
public void method(TreeNode root){
if(root==null) return;
method(root.left);
if(pre==-1){
pre = root.val;
}else{
ans = Math.min(ans,root.val-pre);
pre = root.val;
}
method(root.right);
}
}
题目二
解法
/**
* 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 {
int ans = 0;
public int findTilt(TreeNode root) {
method(root);
return ans;
}
public int method(TreeNode root){
if(root==null) return 0;
int l = method(root.left);
int r = method(root.right);
ans += Math.abs(l-r);
return l+r+root.val;
}
}
题目三
解法
/**
* 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 root, TreeNode subRoot) {
return dfs(root,subRoot);
}
public boolean dfs(TreeNode root, TreeNode subRoot){
if(root==null) return false;
return cheack(root,subRoot)||dfs(root.left,subRoot)||dfs(root.right,subRoot);
}
public boolean cheack(TreeNode root, TreeNode subRoot){
if(root==null&&subRoot==null) return true;
if(root==null||subRoot==null||root.val!=subRoot.val) return false;
return cheack(root.left,subRoot.left)&&cheack(root.right,subRoot.right);
}
}
题目四
解法
/**
* 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 isSameTree(TreeNode p, TreeNode q) {
if(p==null&&q==null) return true;
if(p==null||q==null||q.val!=p.val) return false;
return isSameTree(p.left,q.left)&&isSameTree(p.right,q.right);
}
}
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