By Allison Lampert and Randall Palmer MONTREAL/OTTAWA (Reuters) - Canada's Prime Minister-designate Justin Trudeau, who came from behind to trounce his Conservative rivals and snatch a majority mandate, now has to deliver on pledges from tackling climate change to boosting the economy and legalizing marijuana. Trudeau, who campaigned on a promise of change, toppled Stephen Harper's Conservatives on Monday, giving him the freedom to start implementing his campaign pledges largely unimpeded. The 43-year-old son of former prime minister Pierre Trudeau swept to victory with 39.5 percent of the popular vote in an election that saw the highest voter turnout since 1993.