Merge pull request halfrost#180 from ashtishad/master

Update 031-Next Permutation and 048. Rotate Image

Author: halfrost

leetcode/0031.Next-Permutation/31. Next Permutation.go

@@ -1,31 +1,55 @@
+// https://leetcode.com/problems/next-permutation/discuss/1554932/Go-Submission-with-Explanation
+// Time O(N) , Space: O(1)
+
 package leetcode
 
+// [2,(3),6,5,4,1] -> 2,(4),6,5,(3),1 -> 2,4, 1,3,5,6
 func nextPermutation(nums []int) {
-	i, j := 0, 0
-	for i = len(nums) - 2; i >= 0; i-- {
-		if nums[i] < nums[i+1] {
-			break
-		}
+	var n = len(nums)
+	var pIdx = checkPermutationPossibility(nums)
+	if pIdx == -1 {
+		reverse(nums, 0, n-1)
+		return
 	}
-	if i >= 0 {
-		for j = len(nums) - 1; j > i; j-- {
-			if nums[j] > nums[i] {
-				break
-			}
+
+	var rp = len(nums) - 1
+	// start from right most to leftward,find the first number which is larger than PIVOT
+	for rp > 0 {
+		if nums[rp] > nums[pIdx] {
+			swap(nums, pIdx, rp)
+			break
+		} else {
+			rp--
 		}
-		swap(&nums, i, j)
 	}
-	reverse(&nums, i+1, len(nums)-1)
+	// Finally, Reverse all elements which are right from pivot
+	reverse(nums, pIdx+1, n-1)
 }
 
-func reverse(nums *[]int, i, j int) {
-	for i < j {
-		swap(nums, i, j)
-		i++
-		j--
+func swap(nums []int, i, j int) {
+	nums[i], nums[j] = nums[j], nums[i]
+}
+
+func reverse(nums []int, s int, e int) {
+	for s < e {
+		swap(nums, s, e)
+		s++
+		e--
 	}
 }
 
-func swap(nums *[]int, i, j int) {
-	(*nums)[i], (*nums)[j] = (*nums)[j], (*nums)[i]
+// checkPermutationPossibility returns 1st occurrence Index where
+// value is in decreasing order(from right to left)
+// returns -1 if not found(it's already in its last permutation)
+func checkPermutationPossibility(nums []int) (idx int) {
+	// search right to left for 1st number(from right) that is not in increasing order
+	var rp = len(nums) - 1
+	for rp > 0 {
+		if nums[rp-1] < nums[rp] {
+			idx = rp - 1
+			return idx
+		}
+		rp--
+	}
+	return -1
 }

leetcode/0048.Rotate-Image/48. Rotate Image.go

@@ -1,17 +1,45 @@
+// Time: O(N) Space: O(1)
+
 package leetcode
 
 func rotate(matrix [][]int) {
-	length := len(matrix)
-	// rotate by diagonal 对角线变换
-	for i := 0; i < length; i++ {
-		for j := i + 1; j < length; j++ {
-			matrix[i][j], matrix[j][i] = matrix[j][i], matrix[i][j]
-		}
+	n := len(matrix)
+	if n == 1 {
+		return
 	}
-	// rotate by vertical centerline 竖直轴对称翻转
-	for i := 0; i < length; i++ {
-		for j := 0; j < length/2; j++ {
-			matrix[i][j], matrix[i][length-j-1] = matrix[i][length-j-1], matrix[i][j]
+	/* rotate clock-wise = 1. transpose matrix => 2. reverse(matrix[i])
+
+	1   2  3  4      1   5  9  13        13  9  5  1
+	5   6  7  8  =>  2   6  10 14  =>    14  10 6  2
+	9  10 11 12      3   7  11 15        15  11 7  3
+	13 14 15 16      4   8  12 16        16  12 8  4
+
+	*/
+
+	for i := 0; i < n; i++ {
+		// transpose, i=rows, j=columns
+		// j = i+1, coz diagonal elements didn't change in a square matrix
+		for j := i + 1; j < n; j++ {
+			swap(matrix, i, j)
 		}
+		// reverse each row of the image
+		matrix[i] = reverse(matrix[i])
+	}
+}
+
+// swap changes original slice's i,j position
+func swap(nums [][]int, i, j int) {
+	nums[i][j], nums[j][i] = nums[j][i], nums[i][j]
+}
+
+// reverses a row of image, matrix[i]
+func reverse(nums []int) []int {
+	var lp, rp = 0, len(nums) - 1
+
+	for lp < rp {
+		nums[lp], nums[rp] = nums[rp], nums[lp]
+		lp++
+		rp--
 	}
+	return nums
 }