week7 work

70works/week7/work/day50_547_frind_circles.js

+/**
+ * @param {number[][]} M
+ * @return {number}
+ */
+var findCircleNum = function(M) {
+    if(M.length == 0) return;
+    let n = M.length;
+    let count =n;
+    let parent = new Array(n);
+    for(let i =0; i < n; i++) {
+        parent[i] = i;
+    }
+    const onion = (p, q) => {
+        let rootP = find(p);
+        let rootQ = find(q);
+        if (rootP === rootQ)
+            return;
+        parent[rootP] = rootQ;
+        count--;
+    }
+    const find = (p) => {
+        while(p !== parent[p]) {
+            parent[p] = parent[parent[p]];
+            p = parent[p];
+        }
+        return p;
+    }
+    for(let i =0; i < n; i++) {
+        for(let j =0; j < n; j++) {
+            if(M[i][j] === 1) {
+                onion(i, j);
+            }
+        }
+    }
+    return count;
+};
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70works/week7/work/day53_51_208_nQueens.cpp

+class Solution {
+public:
+    vector<vector<string>> solveNQueens(int n) {
+        vector<vector<string>> res;
+        vector<string> chessBoard(n, string(n, '.'));
+        dfs(n, 0, res, chessBoard);
+        return res;
+    }
+    void dfs(int n, int row, vector<vector<string>> & res,  vector<string>& chessBoard) {
+        if (row == n) {
+            res.push_back(chessBoard);
+            return;
+        }
+        for(int col = 0; col < n; col++) {
+            if (isValid(row, col, n, chessBoard)) {
+                chessBoard[row][col] = 'Q';
+                dfs(n, row+1, res, chessBoard);
+                chessBoard[row][col] = '.';
+            }
+        }
+    }
+    bool isValid(int row, int col, int n, vector<string>&chessBoard) {
+        
+        // 检查固定列每一行上是否有queen
+        for(int i = 0 ; i < row; i++) {
+            if(chessBoard[i][col] == 'Q'){
+                return false;
+            }
+        }
+        // 左上对角检测是否有queen
+        for(int i = row -1, j = col - 1; i >= 0 && j >= 0; i--, j--) {
+            if (chessBoard[i][j] == 'Q') {
+                return false;
+            }
+        }
+
+        //右上对角检测是否有queen 
+        for(int i = row - 1,j = col + 1; i >= 0 && j < n; i--, j++) {
+            if (chessBoard[i][j] == 'Q') {
+                return false;
+            }
+        }
+
+        return true;
+    }
+};
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70works/week7/work/day56_127_word-ladder.cpp

+#include<string>
+#include<vector>
+#include<unordered_set>
+#include<iostream>
+using namespace std;
+class Solution {
+public:
+    int ladderLength(string beginWord, string endWord, vector<string>& wordList) {
+        unordered_set<string> wordSet(wordList.begin(), wordList.end());
+        if (wordSet.count(endWord) == 0) return 0;
+        unordered_set<string> upToDown({beginWord});
+        unordered_set<string>downToUp({endWord});
+        int list_size = wordList.size();
+        int word_size = beginWord.size();
+        vector<int>flag(list_size, 0);
+        int res =2;
+        while(!upToDown.empty()) {
+            unordered_set<string>next;
+            for( auto & word : upToDown) {
+                for(int i =0; i < list_size; i++) {
+                    int count = 0; // count是用来计算beginword和当前比较的string不同的字符个数
+                    for(int j = 0; j < word_size; j++) {
+                        if (word[j] != wordList[i][j]) {
+                            count++;
+                        }
+                        if(count > 1) break; //如果不同的字符个数大于一个以上就跳出循环
+                    }
+                    if (count == 1) { // 题目要求每次只能变一个元素，所以不同字符的个数只能为一，满足条件，才能往下判断
+                        if (downToUp.count(wordList[i]) != 0) return res;
+                        if (flag[i] == 0)
+                        next.insert(wordList[i]);
+                        flag[i] = 1;
+                    }
+                }
+            }
+            res++;
+            upToDown = next;
+            if (upToDown.size() >  downToUp.size()) swap(upToDown, downToUp);
+        }
+        return 0;
+    }
+};
+
+int main() {
+
+    Solution s;
+    int res;
+    string beginWord = "hit";
+    string endWord = "cog";
+    vector<string>wordlist = {"hot","dot","dog","lot","log","cog"};
+    res = s.ladderLength(beginWord, endWord, wordlist);
+    cout<<res<<endl;
+    return 0;
+}
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70works/week7/work/第7周学习总结.md

+#### 第7周学习总结
+#### 第7周学习总结
+
+- 学习内容
+
+  - 字典树；tri树，
+
+    - 题型 
+      - tri树的构建，和删除，增加元素操作
+
+  - 并查集
+
+    - 题型
+      - [547. 朋友圈](https://leetcode-cn.com/problems/friend-circles/)
+
+  - 启发式搜索
+
+    - A*搜索
+
+    - 双向BFS
+
+      - 总结：双向bfs和普通的bfs搜索差在，双向bfs不仅会从前到后搜索，还会从后往前搜索，直到一个节点被访问了两次，一般就可以得出结果了
+
+      - 题型：
+
+        - #### [127. 单词接龙](https://leetcode-cn.com/problems/word-ladder/)
+
+    - 剪枝
+
+  - 红黑树，平衡二叉树
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