力扣---2020.3.4

994. 腐烂的橘子

class Solution {
    int[] dr = new int[]{-1, 0, 1, 0};
    int[] dc = new int[]{0, -1, 0, 1};

    public int orangesRotting(int[][] grid) {
        int R = grid.length, C = grid[0].length;

        // queue : all starting cells with rotten oranges
        Queue<Integer> queue = new ArrayDeque();
        Map<Integer, Integer> depth = new HashMap();
        for (int r = 0; r < R; ++r)
            for (int c = 0; c < C; ++c)
                if (grid[r][c] == 2) {
                    int code = r * C + c;
                    queue.add(code);
                    depth.put(code, 0);
                }

        int ans = 0;
        while (!queue.isEmpty()) {
            int code = queue.remove();
            int r = code / C, c = code % C;
            for (int k = 0; k < 4; ++k) {
                int nr = r + dr[k];
                int nc = c + dc[k];
                if (0 <= nr && nr < R && 0 <= nc && nc < C && grid[nr][nc] == 1) {
                    grid[nr][nc] = 2;
                    int ncode = nr * C + nc;
                    queue.add(ncode);
                    depth.put(ncode, depth.get(code) + 1);
                    ans = depth.get(ncode);
                }
            }
        }

        for (int[] row: grid)
            for (int v: row)
                if (v == 1)
                    return -1;
        return ans;

    }
}

class Solution {
   public int orangesRotting(int[][] grid) {
    int M = grid.length;
    int N = grid[0].length;
    Queue<int[]> queue = new LinkedList<>();

    int count = 0; // count 表示新鲜橘子的数量
    for (int r = 0; r < M; r++) {
        for (int c = 0; c < N; c++) {
            if (grid[r][c] == 1) {
                count++;
            } else if (grid[r][c] == 2) {
                queue.add(new int[]{r, c});
            }
        }
    }

    int round = 0; // round 表示腐烂的轮数，或者分钟数
    while (count > 0 && !queue.isEmpty()) {
        round++;
        int n = queue.size();
        for (int i = 0; i < n; i++) {
            int[] orange = queue.poll();
            int r = orange[0];
            int c = orange[1];
            if (r-1 >= 0 && grid[r-1][c] == 1) {
                grid[r-1][c] = 2;
                count--;
                queue.add(new int[]{r-1, c});
            }
            if (r+1 < M && grid[r+1][c] == 1) {
                grid[r+1][c] = 2;
                count--;
                queue.add(new int[]{r+1, c});
            }
            if (c-1 >= 0 && grid[r][c-1] == 1) {
                grid[r][c-1] = 2;
                count--;
                queue.add(new int[]{r, c-1});
            }
            if (c+1 < N && grid[r][c+1] == 1) {
                grid[r][c+1] = 2;
                count--;
                queue.add(new int[]{r, c+1});
            }
        }
    }

    if (count > 0) {
        return -1;
    } else {
        return round;
    }
    }
}

617. 合并二叉树

//递归
class Solution {
    public TreeNode mergeTrees(TreeNode t1, TreeNode t2) {

        //2.如果t1为空，则返回t2；反之，返回t1
        if(t1==null) return t2;
        if(t2==null) return t1;
        //3.if  t1.left 和 t2.left 同时不为空，则 t1.val = t1.val + t2.val
        t1.val += t2.val;
        t1.left = mergeTrees(t1.left,t2.left);
        t1.right = mergeTrees(t1.right,t2.right);
        return t1;
    }
}

//迭代
public class Solution {
    public TreeNode mergeTrees(TreeNode t1, TreeNode t2) {
        if (t1 == null)
            return t2;
        Stack < TreeNode[] > stack = new Stack < > ();
        stack.push(new TreeNode[] {t1, t2});
        while (!stack.isEmpty()) {
            TreeNode[] t = stack.pop();
            if (t[0] == null || t[1] == null) {
                continue;
            }
            t[0].val += t[1].val;
            if (t[0].left == null) {
                t[0].left = t[1].left;
            } else {
                stack.push(new TreeNode[] {t[0].left, t[1].left});
            }
            if (t[0].right == null) {
                t[0].right = t[1].right;
            } else {
                stack.push(new TreeNode[] {t[0].right, t[1].right});
            }
        }
        return t1;
    }
}

976. 三角形的最大周长

class Solution {
    public int largestPerimeter(int[] A) {
        Arrays.sort(A);
        for(int i = A.length - 1; i >= 2; i--) {
            int a = A[i];
            int b = A[i - 1];
            int c = A[i - 2];
            if(a < b + c){
                return a + b + c;
            }
        }
        return 0;
    }
}

class Solution {
    public int largestPerimeter(int[] A) {
        Arrays.sort(A);
        for (int i = A.length - 3; i >= 0; --i)
            if (A[i] + A[i+1] > A[i+2])
                return A[i] + A[i+1] + A[i+2];
        return 0;
    }
}

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