https://leetcode.com/problems/two-sum-less-than-k https://www.lintcode.com/problem/two-sum-less-than-or-equal-to-target https://www.lintcode.com/problem/two-sum-greater-than-target https://www.lintcode.com/problem/two-sum-difference-equals-to-target https://www.lintcode.com/problem/k-difference https://leetcode.com/problems/two-sum-bsts

https://leetcode.com/problems/two-sum-iii-data-structure-design https://leetcode.com/problems/two-sum-iv-input-is-a-bst

https://leetcode.com/problems/3sum-smaller