1-leetcode200. 岛屿数量
注意:×
- 蛮巧妙的做法,直接在读取到
1
的时候给res
的值+1
,然后深度优先搜索把所有相邻的陆地全部改为海洋 - 注意
dfs
里面的范围判断,[0, **length-1]**
- length-1
- length-1
- length-1
public int numIslands(char[][] grid) {
int res = 0;
int n = grid.length;;
int m = grid[0].length;
for (int i = 0; i < n; i++) {
for (int j = 0; j < m; j++) {
if (grid[i][j] == '1'){
res++;
dfs(grid, i, j);
}
}
}
return res;
}
private void dfs(char[][] grid, int i, int j) {
if (i<0 || i>=grid.length || j<0 || j>=grid[0].length){
return;
}
if (grid[i][j] == '0'){
return;
}
grid[i][j] = '0';
dfs(grid, i+1, j);
dfs(grid, i, j+1);
dfs(grid, i-1, j);
dfs(grid, i, j-1);
}
leetcode994. 腐烂的橘子
注意:×
- 没写,看着考的太少,懒得写了
3-leetcode207. 课程表
注意:××
- 花了很多时间,注意的地方太多了
- 创建
List<Integer>[] graph
的时候,Labuladong说写代码的时候大部分都是这种邻接表的格式,先创建一个LinkedList类型的数组,然后一定要注意,是graph[i] = new LinkedList<>();
- 第二个注意的地方是有
hasVisited
和onPath
两种记录,onPath
的记录我能理解,hasVisited
的记录还需要思考一下,主要的目的就是解决重复搜索
boolean hasCycle = false;
boolean[] hasVisited;
boolean[] onPath;
public boolean canFinish(int numCourses, int[][] prerequisites) {
List<Integer>[] graph = buildGraph(numCourses, prerequisites);
hasVisited = new boolean[numCourses];
onPath = new boolean[numCourses];
for (int i = 0; i < numCourses; i++) {
traverse(graph, i);
}
return !hasCycle;
}
private void traverse(List<Integer>[] graph, int i) {
if (onPath[i]){
hasCycle = true;
}
if (hasCycle || hasVisited[i]){
return;
}
hasVisited[i] = true;
onPath[i] = true;
for (Integer integer : graph[i]) {
traverse(graph, integer);
}
onPath[i] = false;
}
private List<Integer>[] buildGraph(int numCourses, int[][] prerequisites) {
List<Integer>[] graph = new LinkedList[numCourses];
for (int i = 0; i < graph.length; i++) {
graph[i] = new LinkedList<>();
}
for (int[] prerequisite : prerequisites) {
int from = prerequisite[1];
int to = prerequisite[0];
graph[from].add(to);
}
return graph;
}
4-leetcode208. 实现 Trie (前缀树)
注意:×
- 看了题解再写,感觉还是有点压力
- 注意isEnd的调用
class Trie {
class TrieNode{
boolean isEnd;
TrieNode[] nodes;
public TrieNode(){
isEnd = false;
nodes = new TrieNode[26];
}
}
private TrieNode root;
public Trie() {
root = new TrieNode();
}
public void insert(String word) {
TrieNode node = root;
for (char c : word.toCharArray()) {
if (node.nodes[c-'a'] == null){
node.nodes[c-'a'] = new TrieNode();
}
node = node.nodes[c-'a'];
}
node.isEnd = true;
}
public boolean search(String word) {
TrieNode node = root;
for (char c : word.toCharArray()) {
if (node.nodes[c-'a'] == null){
return false;
}
node = node.nodes[c-'a'];
}
return node.isEnd;
}
public boolean startsWith(String prefix) {
TrieNode node = root;
for (char c : prefix.toCharArray()) {
if (node.nodes[c-'a'] == null){
return false;
}
node = node.nodes[c-'a'];
}
return true;
}
}