feat(python): solutions

Author: hongbo-miao

JavaScript/0054. Spiral Matrix.js

@@ -68,7 +68,6 @@ const spiralOrder = (matrix) => {
       y += dirs[dir][1];
       res.push(matrix[x][y]);
     }
-
     steps[dir % 2]--;
     dir = (dir + 1) % 4;
   }

Python/0054. Spiral Matrix.py

@@ -0,0 +1,63 @@
+# Given an m x n matrix, return all elements of the matrix in spiral order.
+#
+# Example 1:
+#
+# Input: matrix = [[1,2,3],[4,5,6],[7,8,9]]
+# Output: [1,2,3,6,9,8,7,4,5]
+#
+# Example 2:
+#
+# Input: matrix = [[1,2,3,4],[5,6,7,8],[9,10,11,12]]
+# Output: [1,2,3,4,8,12,11,10,9,5,6,7]
+#
+# Constraints:
+#
+# m == matrix.length
+# n == matrix[i].length
+# 1 <= m, n <= 10
+# -100 <= matrix[i][j] <= 100
+
+
+# Directions
+# Similar
+# 54. Spiral Matrix
+# 59. Spiral Matrix II
+#
+# https://leetcode.com/problems/spiral-matrix/discuss/20573/A-concise-C++-implementation-based-on-Directions
+#
+# When traversing the matrix in the spiral order, at any time we follow one out of the following four directions:
+# RIGHT DOWN LEFT UP. Suppose we are working on a 5 x 3 matrix as such:
+# 0  1  2  3  4  5
+#    6  7  8  9 10
+#   11 12 13 14 15
+#
+# Imagine a cursor starts off at (0, -1), i.e. the position at '0', then we can achieve the spiral order by doing
+# the following:
+# 1. Go right 5 times
+# 2. Go down 2 times
+# 3. Go left 4 times
+# 4. Go up 1 times.
+# 5. Go right 3 times
+# 6. Go down 0 times -> quit
+#
+# Notice that the directions we choose always follow the order 'right -> down -> left -> up', and for horizontal
+# movements, the number of shifts follows: { 5, 4, 3 }, and vertical movements follows { 2, 1, 0 }.
+# Thus, we can make use of a direction matrix that records the offset for all directions, then an array of two
+# elements that stores the number of shifts for horizontal and vertical movements, respectively. This way, we really
+# just need one for loop instead of four.
+class Solution:
+    def spiralOrder(self, matrix: List[List[int]]) -> List[int]:
+        dirs = [(0, 1), (1, 0), (0, -1), (-1, 0)]
+        steps = [len(matrix[0]), len(matrix) - 1]
+        dir = 0
+        x = 0
+        y = -1
+        res = []
+        while steps[dir % 2] > 0:
+            for _ in range(steps[dir % 2]):
+                x += dirs[dir][0]
+                y += dirs[dir][1]
+                res.append(matrix[x][y])
+            steps[dir % 2] -= 1
+            dir = (dir + 1) % 4
+        return res

Python/0055. Jump Game.py

@@ -0,0 +1,31 @@
+# You are given an integer array nums. You are initially positioned at the array's first index, and each element in the array represents your maximum jump length at that position.
+# Return true if you can reach the last index, or false otherwise.
+#
+# Example 1:
+#
+# Input: nums = [2,3,1,1,4]
+# Output: true
+# Explanation: Jump 1 step from index 0 to 1, then 3 steps to the last index.
+#
+# Example 2:
+#
+# Input: nums = [3,2,1,0,4]
+# Output: false
+# Explanation: You will always arrive at index 3 no matter what. Its maximum jump length is 0, which makes it impossible to reach the last index.
+#
+# Constraints:
+#
+# 1 <= nums.length <= 10^4
+# 0 <= nums[i] <= 10^5
+
+
+# Time O(n)
+# Space O(1)
+class Solution:
+    def canJump(self, nums: List[int]) -> bool:
+        max_reach = 0
+        for i, n in enumerate(nums):
+            if max_reach < i:  # max_reach cannot reach position i
+                return False
+            max_reach = max(max_reach, i + n)
+        return True

Python/0059. Spiral Matrix II.py

@@ -0,0 +1,54 @@
+# Given a positive integer n, generate an n x n matrix filled with elements from 1 to n2 in spiral order.
+#
+# Example 1:
+#
+# Input: n = 3
+# Output: [[1,2,3],[8,9,4],[7,6,5]]
+#
+# Example 2:
+#
+# Input: n = 1
+# Output: [[1]]
+#
+# Constraints:
+#
+# 1 <= n <= 20
+
+
+# Directions
+# Similar
+# 54. Spiral Matrix
+# 59. Spiral Matrix II
+#
+# When traversing the matrix in the spiral order, at any time we follow one out of the following four directions:
+# RIGHT DOWN LEFT UP. Suppose we are working on a 5 x 3 matrix as such:
+# 0  1  2  3  4  5
+#    6  7  8  9 10
+#   11 12 13 14 15
+#
+# Imagine a cursor starts off at (0, -1), i.e. the position at '0', then we can achieve the spiral order by doing
+# the following:
+# 1. Go right 5 times
+# 2. Go down 2 times
+# 3. Go left 4 times
+# 4. Go up 1 times.
+# 5. Go right 3 times
+# 6. Go down 0 times -> quit
+class Solution:
+    def generateMatrix(self, n: int) -> List[List[int]]:
+        matrix = [[None] * n for _ in range(n)]
+        dirs = [[0, 1], [1, 0], [0, -1], [-1, 0]]
+        steps = [n, n - 1]
+        dir = 0
+        x = 0
+        y = -1
+        num = 1
+        while steps[dir % 2] > 0:
+            for _ in range(steps[dir % 2]):
+                x += dirs[dir][0]
+                y += dirs[dir][1]
+                matrix[x][y] = num
+                num += 1
+            steps[dir % 2] -= 1
+            dir = (dir + 1) % 4
+        return matrix