一、找树左下角的值
题目链接：513. 找树左下角的值
/**
* <pre>
* 1.广搜，每一层遍历，找到最左边的节点记录下值
* 2.深搜，找到最深的一层就将结果记录下来，先遍历的是左节点，所以右节点如果同样高度则不会覆盖掉左节点的值，保证了最终找到的是最左边的节点
* </pre>
*
* @author <a href="https://github.com/Ken-Chy129">Ken-Chy129</a>
* @date 2023/1/16 13:44
*/
public class 找树左下角的值513 {
public int findBottomLeftValue(TreeNode root) {
Queue<TreeNode> queue = new LinkedList<>();
queue.offer(root);
int ans = root.val;
boolean flag = true; // 此处也可以不使用flag，而是每次offer入队的时候先入right节点，那么最后一次poll出来的节点就是当层最左边的节点
while (!queue.isEmpty()) {
int size = queue.size();
for (int i=0; i<size; i++) {
TreeNode poll = queue.poll();
if (poll.left != null) {
queue.offer(poll.left);
if (flag) {
ans = poll.left.val;
flag = false;
}
}
if (poll.right != null) {
queue.offer(poll.right);
if (flag) {
ans = poll.right.val;
flag = false;
}
}
}
flag = true;
}
return ans;
}
int res = 0;
int maxHeight = -1;
public int findBottomLeftValue2(TreeNode root) {
dfs(root, 0);
return res;
}
public void dfs(TreeNode root, int height) {
if (root.left == null && root.right == null) {
if (height > maxHeight) {
maxHeight = height;
res = root.val;
}
return;
}
if (root.left != null) {
dfs(root.left, height+1);
}
if (root.right != null) {
dfs(root.right, height+1);
}
}
}
二、路径总和
题目链接：112. 路径总和
/**
* <pre>
* 1.深搜
* 2.广搜
* 节点的值有可能是负数，所以不能大于targetSum就剪枝
* </pre>
*
* @author <a href="https://github.com/Ken-Chy129">Ken-Chy129</a>
* @date 2023/1/16 15:19
*/
public class 路径总和112 {
public boolean hasPathSum(TreeNode root, int targetSum) {
if (root == null) {
return false;
}
return dfs(root, targetSum, root.val);
}
public boolean dfs(TreeNode root, int targetSum, int nowSum) {
if (root.left == null && root.right == null) {
return nowSum == targetSum;
}
boolean rs1=false, rs2=false;
if (root.left != null) {
rs1 = dfs(root.left, targetSum, nowSum+root.left.val);
}
if (root.right != null) {
rs2 = dfs(root.right, targetSum, nowSum+root.right.val);
}
return rs1 || rs2;
}
public boolean hasPathSum2(TreeNode root, int targetSum) {
if (root == null) {
return false;
}
Queue<TreeNode> nodeQueue = new LinkedList<>();
Queue<Integer> sumQueue = new LinkedList<>();
nodeQueue.offer(root);
sumQueue.offer(root.val);
while (!nodeQueue.isEmpty()) {
int size = nodeQueue.size();
for (int i=0; i<size; i++) {
TreeNode node = nodeQueue.poll();
Integer sum = sumQueue.poll();
if (node.left == null && node.right == null) {
if (sum == targetSum) {
return true;
}
}
if (node.left != null) {
nodeQueue.offer(node.left);
sumQueue.offer(sum+node.left.val);
}
if (node.right != null) {
nodeQueue.offer(node.right);
sumQueue.offer(sum+node.right.val);
}
}
}
return false;
}
}
三、从前序与中序遍历序列构造二叉树
题目链接：105. 从前序与中序遍历序列构造二叉树
/**
* <pre>
* 递归：从前往后依次取出前缀遍历数组的值，以其作为分割点，不断将中序数组分割为左右子树
* </pre>
*
* @author <a href="https://github.com/Ken-Chy129">Ken-Chy129</a>
* @date 2023/1/16 17:24
*/
public class 从前序与中序遍历序列构造二叉树105 {
int pre_idx = 0;
int[] preorder;
int[] inorder;
Map<Integer, Integer> idx_map = new HashMap<>();
public TreeNode buildTree(int[] preorder, int[] inorder) {
this.preorder = preorder;
this.inorder = inorder;
int index = 0;
for (int val : inorder) {
idx_map.put(val, index++);
}
return build(0, inorder.length-1);
}
public TreeNode build(int in_left, int in_right) {
if (in_left > in_right) {
return null;
}
int val = preorder[pre_idx++];
TreeNode root = new TreeNode(val);
Integer index = idx_map.get(val);
root.left = build(in_left, index-1);
root.right = build(index+1, in_right);
return root;
}
}
四、从中序与后序遍历序列构造二叉树
题目链接：106.从中序与后序遍历序列构造二叉树
/**
* <pre>
* 递归：从后往前依次取出前缀遍历数组的值，以其作为分割点，不断将中序数组分割为右左子树
* </pre>
*
* @author <a href="https://github.com/Ken-Chy129">Ken-Chy129</a>
* @date 2023/1/16 16:21
*/
public class 从中序与后序遍历序列构造二叉树106 {
int post_idx;
int[] postorder;
int[] inorder;
Map<Integer, Integer> idx_map = new HashMap<>();
public TreeNode buildTree(int[] inorder, int[] postorder) {
this.inorder = inorder;
this.postorder = postorder;
post_idx = postorder.length - 1;
int index = 0;
for (int val : inorder) {
idx_map.put(val, index++);
}
return build(0, inorder.length-1);
}
public TreeNode build(int in_left, int in_right) {
// 如果这里没有节点构造二叉树了，就结束
if (in_left > in_right) {
return null;
}
// 选择 post_idx 位置的元素作为当前子树根节点
int root_val = postorder[post_idx];
TreeNode root = new TreeNode(root_val);
// 根据 root 所在位置分成左右两棵子树
int index = idx_map.get(root_val);
// 下标减一
post_idx--;
// 先构造右子树，因为从后取后序遍历数组的值，去完根节点后接着最开始取到的就是最右边的节点
root.right = build(index + 1, in_right);
// 构造左子树
root.left = build(in_left, index - 1);
return root;
}
}
五、最大二叉树
题目链接：654. 最大二叉树
/**
* <pre>
* 递归
* 单调栈
* </pre>
*
* @author <a href="https://github.com/Ken-Chy129">Ken-Chy129</a>
* @date 2023/1/17 11:45
*/
public class 最大二叉树654 {
public TreeNode constructMaximumBinaryTree(int[] nums) {
if (nums.length == 0) {
return null;
}
TreeNode root = new TreeNode();
int max = nums[0], index=0;
for (int i=1; i<nums.length; i++) {
if (nums[i] > max) {
index = i;
max = nums[i];
}
}
root.val = max;
root.left = constructMaximumBinaryTree(Arrays.copyOfRange(nums, 0, index));
root.right = constructMaximumBinaryTree(Arrays.copyOfRange(nums, index+1, nums.length));
return root;
}
}
六、合并二叉树
题目链接：617. 合并二叉树
/**
* <pre>
* 1.深搜
* 2.广搜
* </pre>
*
* @author <a href="https://github.com/Ken-Chy129">Ken-Chy129</a>
* @date 2023/1/17 12:23
*/
public class 合并二叉树617 {
public TreeNode mergeTrees(TreeNode root1, TreeNode root2) {
if (root1 == null) {
return root2;
}
if (root2 == null) {
return root1;
}
TreeNode merged = new TreeNode(root1.val + root2.val);
merged.left = mergeTrees(root1.left, root2.left);
merged.right = mergeTrees(root1.right, root2.right);
return merged;
}
public TreeNode mergeTrees2(TreeNode root1, TreeNode root2) {
if (root1 == null) {
return root2;
}
if (root2 == null) {
return root1;
}
Queue<TreeNode> treeQueue1 = new LinkedList<>();
Queue<TreeNode> treeQueue2 = new LinkedList<>();
Queue<TreeNode> mergedQueue = new LinkedList<>();
treeQueue1.offer(root1);
treeQueue2.offer(root2);
TreeNode ans = new TreeNode(root1.val + root2.val);
mergedQueue.offer(ans);
while (!treeQueue1.isEmpty() || !treeQueue2.isEmpty()) {
TreeNode treeNode1 = treeQueue1.poll();
TreeNode treeNode2 = treeQueue2.poll();
TreeNode root = mergedQueue.poll();
TreeNode mergedNode1, mergedNode2;
if (treeNode1.left == null) {
mergedNode1 = treeNode2.left;
} else if (treeNode2.left == null) {
mergedNode1 = treeNode1.left;
} else {
treeQueue1.offer(treeNode1.left);
treeQueue2.offer(treeNode2.left);
mergedNode1 = new TreeNode(treeNode1.left.val + treeNode2.left.val);
mergedQueue.offer(mergedNode1);
}
root.left = mergedNode1;
if (treeNode1.right == null) {
mergedNode2 = treeNode2.right;
} else if (treeNode2.right == null) {
mergedNode2 = treeNode1.right;
} else {
treeQueue1.offer(treeNode1.right);
treeQueue2.offer(treeNode2.right);
mergedNode2 = new TreeNode(treeNode1.right.val + treeNode2.right.val);
mergedQueue.offer(mergedNode2);
}
root.right = mergedNode2;
}
return ans;
}
}