算法-算法|ss dfs&bfs

• 200.岛屿数量—1
• 207.课程表—1
• 547.省份数量
• 695.岛屿的最大面积

200.岛屿数量—1

/**
 * @param {character[][]} grid
 * @return {number}
 */
var numIslands = function (grid) {
  let ans = 0;
  let m = grid.length;
  let n = grid[0].length;
  const dirs = [
    [1, 0],
    [-1, 0],
    [0, 1],
    [0, -1],
  ];
  const checkList = Array(m)
    .fill(0)
    .map(() => Array(n).fill(0));
  for (let i = 0; i < m; i++) {
    for (let j = 0; j < n; j++) {
      if (grid[i][j] === "1" && checkList[i][j] === 0) {
        ans += bfs(i, j);
      }
    }
  }

  function bfs(i, j) {
    let count = 1;
    const queue = [];
    queue.push([i, j]);
    while (queue.length) {
      const [x, y] = queue.pop();
      for (let [dx, dy] of dirs) {
        let nextX = dx + x;
        let nextY = dy + y;
        if (
          nextX >= 0 &&
          nextX < m &&
          nextY >= 0 &&
          nextY < n &&
          grid[nextX][nextY] === "1" &&
          checkList[nextX][nextY] === 0
        ) {
          queue.push([nextX, nextY]);
          checkList[nextX][nextY] = 1;
        }
      }
    }
    return count;
  }
  return ans;
};
numIslands([
  ["1", "1", "1", "1", "0"],
  ["1", "1", "0", "1", "0"],
  ["1", "1", "0", "0", "0"],
  ["0", "0", "0", "0", "0"],
]);

207.课程表—1

// numCourses = 2, prerequisites = [[1,0]]
/**
 * @param {number} numCourses
 * @param {number[][]} prerequisites
 * @return {boolean}
 */
var canFinish = function (numCourses, prerequisites) {
  const map = {};
  const nextMap = {};
  for (let [a, b] of prerequisites) {
    if (map[a] === undefined) {
      map[a] = 0;
    }
    if (nextMap[b] === undefined) {
      nextMap[b] = [];
    }
    map[a] += 1;
    nextMap[b].push(a);
    if (map[b] === undefined) map[b] = 0;
  }
  //
  const queue = [];
  for (let ch in map) {
    if (map[ch] === 0) {
      queue.push(ch);
    }
  }
  while (queue.length) {
    const cur = queue.shift();
    const nextCur = nextMap[cur];
    if (!nextCur) continue;
    for (let ch of nextCur) {
      map[ch] -= 1;
      if (map[ch] === 0) {
        queue.push(ch);
      }
    }
  }
  //   console.log(map);
  return Object.values(map).every((item) => item === 0);
};
canFinish(2, [
  [1, 0],
  [0, 1],
]);

547.省份数量

/**
 * @param {number[][]} isConnected
 * @return {number}
 */
// 思路
// 遍历一层， 对没有检查的可以看作是一个省份
// 递归dfs，对相连的城市都去dfs一下，然后设置对应的checklist为1，
// dfs的作用就是使相连的城市的checklist都为1
var findCircleNum = function (isConnected) {
  const n = isConnected.length;
  const checklist = Array(n).fill(0);
  let ans = 0;
  for (let i = 0; i < n; i++) {
    if (checklist[i] === 0) {
      dfs(i);
      ans += 1;
    }
  }
  //   console.log(ans);
  return ans;
  function dfs(idx) {
    checklist[idx] = 1;
    for (let j = 0; j < n; j++) {
      if (j !== idx && checklist[j] === 0 && isConnected[idx][j] === 1) {
        dfs(j);
      }
    }
  }
};
findCircleNum([
  [1, 0, 0],
  [0, 1, 0],
  [0, 0, 1],
]);
// [[1,0,0],[0,1,0],[0,0,1]]

695.岛屿的最大面积

/**
 * @param {number[][]} grid
 * @return {number}
 */
// 思路
// dfs的返回值 应该怎么定义
// 什么条件下返回
// 合格返回 1
// 不合格返回0
var maxAreaOfIsland = function (grid) {
  const m = grid.length;
  const n = grid[0].length;
  let ans = 0;
  let dirs = [
    [-1, 0],
    [1, 0],
    [0, 1],
    [0, -1],
  ];
  const checklist = Array(m)
    .fill(0)
    .map(() => Array(n).fill(0));
  for (let i = 0; i < m; i++) {
    for (let j = 0; j < n; j++) {
      if (checklist[i][j] === 0 && grid[i][j] === 1) {
        ans = Math.max(ans, dfs(i, j));
      }
    }
  }
  //   console.log(ans);
  return ans;
  function dfs(x, y) {
    // 不符合条件的return 0
    if (
      x < 0 ||
      x >= m ||
      y < 0 ||
      y >= n ||
      grid[x][y] === 0 ||
      checklist[x][y] === 1
    ) {
      return 0;
    }
    let ans = 1;
    // 符合条件的返回ans
    checklist[x][y] = 1;
    for (let [dx, dy] of dirs) {
      ans += dfs(dx + x, dy + y);
    }
    return ans;
  }
};
maxAreaOfIsland([
  [0, 0, 1, 0, 0, 0, 0, 1, 0, 0, 0, 0, 0],
  [0, 0, 0, 0, 0, 0, 0, 1, 1, 1, 0, 0, 0],
  [0, 1, 1, 0, 1, 0, 0, 0, 0, 0, 0, 0, 0],
  [0, 1, 0, 0, 1, 1, 0, 0, 1, 0, 1, 0, 0],
  [0, 1, 0, 0, 1, 1, 0, 0, 1, 1, 1, 0, 0],
  [0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 1, 0, 0],
  [0, 0, 0, 0, 0, 0, 0, 1, 1, 1, 0, 0, 0],
  [0, 0, 0, 0, 0, 0, 0, 1, 1, 0, 0, 0, 0],
]);
// [[0,0,1,0,0,0,0,1,0,0,0,0,0],[0,0,0,0,0,0,0,1,1,1,0,0,0],[0,1,1,0,1,0,0,0,0,0,0,0,0],[0,1,0,0,1,1,0,0,1,0,1,0,0],[0,1,0,0,1,1,0,0,1,1,1,0,0],[0,0,0,0,0,0,0,0,0,0,1,0,0],[0,0,0,0,0,0,0,1,1,1,0,0,0],[0,0,0,0,0,0,0,1,1,0,0,0,0]]