On the day of the funeral of Barbara Bush, here is a list of all Presidents who were widowed, a total of 11 of 44 Presidents, 3 in office, and 8 out of office, and 4 remarrying, and 2 (Tyler and Wilson) while in office.
John Adams out of office in 1818
James Monroe out of office in 1830
John Tyler in office 1842, remarried in 1844 in office
Millard Fillmore out of office 1853, remarried
Franklin Pierce out of office 1863
Rutherford B. Hayes out of office 1889
Benjamin Harrison in office 1892, remarried
Woodrow Wilson in office 1914, remarried in 1915 in office
Herbert Hoover out of office 1944
Richard Nixon out of office 1993
George H. W. Bush out of office 2018