算法面试题四：两数之和，有效的数独，旋转图像

这里介绍两数之和,有效的数独及旋转图像的个人解决方法

题目一：两数之和

给定一个整数数组 nums 和一个整数目标值 target，请你在该数组中找出 和为目标值 的那 两个 整数，并返回它们的数组下标。

你可以假设每种输入只会对应一个答案。但是，数组中同一个元素不能使用两遍。

你可以按任意顺序返回答案。

示例 1：

输入：nums = [2,7,11,15], target = 9
输出：[0,1]
解释：因为 nums[0] + nums[1] == 9 ，返回 [0, 1] 。

示例 2：

输入：nums = [3,2,4], target = 6
输出：[1,2]

示例 3：

输入：nums = [3,3], target = 6
输出：[0,1]

提示：

2 <= nums.length <= 103
-109 <= nums[i] <= 109
-109 <= target <= 109
只会存在一个有效答案

答案：

/**
 * @param {number[]} nums
 * @param {number} target
 * @return {number[]}
 */
var twoSum = function(nums, target) {
    let arr=[];
    for(let i=0;i<nums.length-1;i++){
        let bind = true;
        let jj = 0;
        for(let j=i+1;j<nums.length;j++){
            if(nums[i]+nums[j] == target){
                bind = false;
                jj = j;
            }
        }
        if(!bind){
            arr.push(i)
            arr.push(jj)
            nums.splice(i,1);
            nums.splice(jj,1);
            i--;
        }
    }
    return arr;
};

题目二：有效的数独

判断一个 9x9 的数独是否有效。只需要根据以下规则，验证已经填入的数字是否有效即可。

1.数字 1-9 在每一行只能出现一次。
2.数字 1-9 在每一列只能出现一次。
3.数字 1-9 在每一个以粗实线分隔的 3x3 宫内只能出现一次。

上图是一个部分填充的有效的数独。

数独部分空格内已填入了数字，空白格用 '.' 表示。

示例 1:

输入:
[
  ["5","3",".",".","7",".",".",".","."],
  ["6",".",".","1","9","5",".",".","."],
  [".","9","8",".",".",".",".","6","."],
  ["8",".",".",".","6",".",".",".","3"],
  ["4",".",".","8",".","3",".",".","1"],
  ["7",".",".",".","2",".",".",".","6"],
  [".","6",".",".",".",".","2","8","."],
  [".",".",".","4","1","9",".",".","5"],
  [".",".",".",".","8",".",".","7","9"]
]
输出: true

示例 2:

输入:
[
  ["8","3",".",".","7",".",".",".","."],
  ["6",".",".","1","9","5",".",".","."],
  [".","9","8",".",".",".",".","6","."],
  ["8",".",".",".","6",".",".",".","3"],
  ["4",".",".","8",".","3",".",".","1"],
  ["7",".",".",".","2",".",".",".","6"],
  [".","6",".",".",".",".","2","8","."],
  [".",".",".","4","1","9",".",".","5"],
  [".",".",".",".","8",".",".","7","9"]
]
输出: false

说明:

一个有效的数独（部分已被填充）不一定是可解的。
只需要根据以上规则，验证已经填入的数字是否有效即可。
给定数独序列只包含数字 1-9 和字符 '.' 。
给定数独永远是 9x9 形式的。

答案：

/**
 * @param {character[][]} board
 * @return {boolean}
 */
var isValidSudoku = function(board) {
    for (let arr of board) {
    let row = []
    for (let c of arr) {
      if (c !== '.') row.push(c);
    }
    let set = new Set(row)
    if (set.size !== row.length) return false;
  }

//   检查每一列
  for (let i = 0; i < 9; i++) {
    let col = []
    board.map( arr => {
      if (arr[i] !== '.') col.push(arr[i])
    })
    let set = new Set(col)
    if (set.size !== col.length) return false;
  }

//   检查每个小方块
  for (let x = 0; x < 9; x += 3) {
    for (let y = 0; y < 9; y += 3) {
      let box = []
      for (let a = x; a < 3 + x; a ++) {
        for (let b = y; b < 3 + y; b ++) {
          if (board[a][b] !== '.') box.push(board[a][b])
        }
      }
      let set = new Set(box)
      if (set.size !== box.length) return false
    }
  }

  return true
};

题目三：旋转图像

给定一个 n × n 的二维矩阵 matrix 表示一个图像。请你将图像顺时针旋转 90 度。

你必须在 原地 旋转图像，这意味着你需要直接修改输入的二维矩阵。请不要 使用另一个矩阵来旋转图像。

示例 1：

输入：matrix = [[1,2,3],[4,5,6],[7,8,9]]
输出：[[7,4,1],[8,5,2],[9,6,3]]

示例 2：

输入：matrix = [[5,1,9,11],[2,4,8,10],[13,3,6,7],[15,14,12,16]]
输出：[[15,13,2,5],[14,3,4,1],[12,6,8,9],[16,7,10,11]]

示例 3：

输入：matrix = [[1]]
输出：[[1]]

示例 4：

输入：matrix = [[1,2],[3,4]]
输出：[[3,1],[4,2]]

提示：

matrix.length == n
matrix[i].length == n
1 <= n <= 20
-1000 <= matrix[i][j] <= 1000

答案：

/**
 * @param {number[][]} matrix
 * @return {void} Do not return anything, modify matrix in-place instead.
 */
var rotate = function(matrix) {
    let length = matrix.length;
    for(let i=0;i<length/2;i++){
        for (let j = i; j < length - 1 - i; j++) {
            let tmp = matrix[i][j];
            matrix[i][j] = matrix[length - 1 - j][i];
            matrix[length - 1 - j][i] = matrix[length - 1 - i][length - 1 - j];
            matrix[length - 1 - i][length - 1 - j] = matrix[j][length - 1 - i];
            matrix[j][length - 1 - i] = tmp;
        }
    }
    return matrix;
};

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