add: my solution for pattern13 challenge3

Pattern13 - Top K Elements/Challenge3/solution.go

@@ -0,0 +1,76 @@
+package challenge3
+
+import "container/heap"
+
+/*
+* Frequency Stack
+Design a class that simulates a Stack data structure, implementing the following two operations:
+1. push(int num): Pushes the number ‘num’ on the stack.
+2. pop(): Returns the most frequent number in the stack. If there is a tie, return the number which was pushed later.
+
+After following push operations: push(1), push(2), push(3), push(2), push(1), push(2), push(5)
+
+1. pop() should return 2, as it is the most frequent number
+2. Next pop() should return 1
+3. Next pop() should return 2
+
+ref: https://leetcode-cn.com/problems/maximum-frequency-stack/
+*/
+
+type FreqStack struct {
+	Stack []int
+	Heap  FreqHeap
+	Map   map[int]*Frequency
+}
+
+func Constructor() FreqStack {
+	fs := FreqStack{}
+	heap.Init(&fs.Heap)
+	fs.Map = make(map[int]*Frequency)
+	return fs
+}
+
+func (this *FreqStack) Push(val int) {
+	this.Stack = append(this.Stack, val)
+	if f, ok := this.Map[val]; ok {
+		f.LastInd = len(this.Stack) - 1
+		f.Times++
+	} else {
+		this.Map[val] = &Frequency{val, 1, len(this.Stack) - 1}
+	}
+	heap.Push(&this.Heap, *this.Map[val])
+}
+
+func (this *FreqStack) Pop() int {
+	x := heap.Pop(&this.Heap).(Frequency).Val
+	this.Map[x].Times--
+	if this.Map[x].Times == 0 {
+		delete(this.Map, x)
+	}
+	return x
+}
+
+type Frequency struct {
+	Val     int
+	Times   int
+	LastInd int
+}
+type FreqHeap []Frequency
+
+func (h FreqHeap) Len() int { return len(h) }
+func (h FreqHeap) Less(i, j int) bool {
+	if h[i].Times != h[j].Times {
+		return h[i].Times > h[j].Times
+	} else {
+		return h[i].LastInd > h[j].LastInd
+	}
+}
+func (h FreqHeap) Swap(i, j int)       { h[i], h[j] = h[j], h[i] }
+func (h *FreqHeap) Push(x interface{}) { *h = append(*h, x.(Frequency)) }
+func (h *FreqHeap) Pop() interface{} {
+	old := *h
+	n := len(old)
+	x := old[n-1]
+	*h = old[:n-1]
+	return x
+}

Pattern13 - Top K Elements/Challenge3/solution_test.go

@@ -0,0 +1,32 @@
+package challenge3
+
+import (
+	"testing"
+)
+
+func TestConstructor(t *testing.T) {
+
+	fs := Constructor()
+	fs.Push(5)
+	fs.Push(7)
+	fs.Push(5)
+	fs.Push(7)
+	fs.Push(4)
+	fs.Push(5)
+	x := fs.Pop()
+	if x != 5 {
+		t.Errorf("pop %v, want %v", x, 5)
+	}
+	x = fs.Pop()
+	if x != 7 {
+		t.Errorf("pop %v, want %v", x, 7)
+	}
+	x = fs.Pop()
+	if x != 5 {
+		t.Errorf("pop %v, want %v", x, 5)
+	}
+	x = fs.Pop()
+	if x != 4 {
+		t.Errorf("pop %v, want %v", x, 4)
+	}
+}