Subtopic: k pointers

思路出发点，分解问题的方式并没有任何改变，start from brute force

• for all the tuples (i, j, k, l), where i in a, j in b, k in c, l in d;

Question 3.0. Given two sorted arrays A and B, if there exist two numbers a from A and b from B, such that a+ b = target

1. 分解A[i] + B[j] ?= target

2. 移动方向？

   1. 当j往左边移动的时候

      1. B[j]减少？

      2. A[i] = target - B[j]增大

      3. i往右移动

• 相向而行的双指针

1. implementation: we cannot miss any possibe j?

Question 3.1 What about 3 sorted arrays? a+ b+ c = target from A, B, C

1. 分解 A[i] + B[j] + C[k] = target

   1. 分解方式：固定k

2. for each K

Question 3.2 What about k sorted arrays?