Leetcode199. 二叉树的右视图 题目:
给定一棵二叉树,想象自己站在它的右侧,按照从顶部到底部的顺序,返回从右侧所能看到的节点值。
示例:
输入: [1,2,3,null,5,null,4]
输出: [1, 3, 4]
思路1(广度优先搜索):
对二叉树进行层次遍历,每层最右边的结点一定是最后被遍历到的。二叉树的层次遍历可以用广度优先搜索实现。
Java AC代码:
import java.util.*;
public class Leetcode199 {
public class TreeNode {
int val;
TreeNode left;
TreeNode right;
TreeNode(int x) {
val = x;
}
}
//广度优先搜索
public List<Integer> rightSideView(TreeNode root) {
Map<Integer, Integer> map = new HashMap<>();
int max_depth = -1;
Queue<TreeNode> nodeQueue = new LinkedList<>();
Queue<Integer> depthQueue = new LinkedList<>();
nodeQueue.add(root);
depthQueue.add(0);
while (!nodeQueue.isEmpty()) {
TreeNode node = nodeQueue.remove();
int depth = depthQueue.remove();
if (node != null) {
max_depth = Math.max(max_depth, depth);
map.put(depth, node.val);
nodeQueue.add(node.left);
nodeQueue.add(node.right);
depthQueue.add(depth + 1);
depthQueue.add(depth + 1);
}
}
List<Integer> rightSideView = new ArrayList<>();
for (int i = 0; i <= max_depth; i++) {
rightSideView.add(map.get(i));
}
return rightSideView;
}
}
思路2(深度优先搜索):
对树进行深度优先搜索,在搜索过程中,我们总是先访问右子树。那么对于每一层来说,我们在这层见到的第一个结点一定是最右边的结点。
Java AC代码:
import java.util.*;
public class Leetcode199 {
public class TreeNode {
int val;
TreeNode left;
TreeNode right;
TreeNode(int x) {
val = x;
}
}
//深度优先搜索
public List<Integer> rightSideView(TreeNode root) {
Map<Integer, Integer> rightmostValueAtDepth = new HashMap<Integer, Integer>();
int max_depth = -1;
Stack<TreeNode> nodeStack = new Stack<TreeNode>();
Stack<Integer> depthStack = new Stack<Integer>();
nodeStack.push(root);
depthStack.push(0);
while (!nodeStack.isEmpty()) {
TreeNode node = nodeStack.pop();
int depth = depthStack.pop();
if (node != null) {
// 维护二叉树的最大深度
max_depth = Math.max(max_depth, depth);
// 如果不存在对应深度的节点我们才插入
if (!rightmostValueAtDepth.containsKey(depth)) {
rightmostValueAtDepth.put(depth, node.val);
}
nodeStack.push(node.left);
nodeStack.push(node.right);
depthStack.push(depth + 1);
depthStack.push(depth + 1);
}
}
List<Integer> rightView = new ArrayList<Integer>();
for (int depth = 0; depth <= max_depth; depth++) {
rightView.add(rightmostValueAtDepth.get(depth));
}
return rightView;
}
}
2020.4.22打卡