Merge two maps

Updated Oct 7, 2026

Merge combines two key sets. On conflict, pick a policy: last map wins, first map wins, or combine values (add counts, concat lists). Size of the result is |keysA ∪ keysB|.

What to focus on

Do not assume iteration order.

Worked examples

a = [4] as {4:2}, b = [1] as {1:8}

a = [4, 1] as {4:1,1:2}, b = [1, 9] as {1:1,9:1}

a = [], b = [4] as {4:2}

Try it

Write merge(a, b, policy). a and b are non-empty maps. policy is "union", "addCounts", or "lastWins". Return the merged map. Do not assume iteration order. a = [4] as {4:2}, b = [1] as {1:8} → {4:2, 1:8}. a = [4, 1] as {4:1,1:2}, b = [1, 9] as {1:1,9:1} → {4:1, 1:3, 9:1}. a = [], b = [4] as {4:2} → {4:2}.

keys are integers. Either map may be empty.