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前言

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一、力扣322. 零钱兑换

class Solution {public int coinChange(int[] coins, int amount) {int[] dp = new int[amount+1];Arrays.fill(dp,Integer.MAX_VALUE);dp[0] = 0;for(int i = 0; i < coins.length; i ++){for(int j = coins[i]; j <= amount; j ++){if(dp[j-coins[i]] != Integer.MAX_VALUE){dp[j] = Math.min(dp[j],dp[j-coins[i]] + 1);//最大值加1就会变成负数了}}}return dp[amount] == Integer.MAX_VALUE ? -1:dp[amount];}
}

二、力扣509. 斐波那契数

class Solution {public int fib(int n) {if(n == 0 || n == 1){return n;}int[] dp = new int[n+1];dp[0] = 0;dp[1] = 1;for(int i = 2; i <= n; i ++){dp[i] = dp[i-1] + dp[i-2];}return dp[n];}
}

三、力扣46. 全排列

class Solution {List<List<Integer>> res = new ArrayList<>();List<Integer> path = new ArrayList<>();boolean[] flag;public List<List<Integer>> permute(int[] nums) {flag = new boolean[nums.length];fun(nums);return res;}public void fun(int[] nums){if(path.size() == nums.length){res.add(new ArrayList<>(path));return;}for(int i = 0; i < nums.length; i ++){if(flag[i] == true){continue;}flag[i] = true;path.add(nums[i]);fun(nums);flag[i] = false;path.remove(path.size()-1);}}
}

四、力扣51. N 皇后

class Solution {List<List<String>> res = new ArrayList<>();public List<List<String>> solveNQueens(int n) {char[][] ch = new char[n][n];for(int i = 0; i < n; i ++){for(int j = 0; j < n; j ++){ch[i][j] = '.';}}fun(ch,0,n);return res;}public void fun(char[][] ch, int depth,int n){if(depth == n){List<String> path = new ArrayList<>();for(char[] temp : ch){path.add(new String(temp));}res.add(path);return;}for(int i = 0; i < n; i ++){if(flag(ch, depth,i,n)){ch[depth][i] = 'Q';fun(ch,depth+1, n);ch[depth][i] = '.';}}}public boolean flag(char[][] ch, int row, int col, int n){//判断行for(int i = 0; i < col;i ++){System.out.println("row = " + row + " __ col = " + i);if(ch[row][i] == 'Q'){return false;}}//判断列for(int i = 0; i < row; i ++){if(ch[i][col] == 'Q'){return false;}}//判断45°for(int i = row, j = col; i >= 0 && j >= 0 ; i --, j --){if(ch[i][j] == 'Q'){return false;}}//判断135度for(int i = row, j = col; i >= 0 && j < n; i --, j ++){if(ch[i][j] == 'Q'){return false;}}return true;}
}

五、力扣52. N 皇后 II

class Solution {int res = 0;public int totalNQueens(int n) {char[][] ch = new char[n][n];for(int i = 0; i < n; i ++){for(int j = 0; j < n; j ++){ch[i][j] = '.';}}fun(ch,0,n);return res;}public void fun(char[][] ch, int depth,int n){if(depth == n){res ++;return;}for(int i = 0; i < n; i ++){if(flag(ch, depth,i,n)){ch[depth][i] = 'Q';fun(ch,depth+1, n);ch[depth][i] = '.';}}}public boolean flag(char[][] ch, int row, int col, int n){//判断行for(int i = 0; i < col;i ++){System.out.println("row = " + row + " __ col = " + i);if(ch[row][i] == 'Q'){return false;}}//判断列for(int i = 0; i < row; i ++){if(ch[i][col] == 'Q'){return false;}}//判断45°for(int i = row, j = col; i >= 0 && j >= 0 ; i --, j --){if(ch[i][j] == 'Q'){return false;}}//判断135度for(int i = row, j = col; i >= 0 && j < n; i --, j ++){if(ch[i][j] == 'Q'){return false;}}return true;}
}