HDU5937 Equation（DFS + 剪枝）
题目 Source http://acm.hdu.edu.cn/showproblem.php?pid=5937 Description Little Ruins is a studious boy, recently he learned addition operation! He was rewarded some number bricks of 1 to 9 and infinity bricks of addition mark '+' and equal mark '='. Now little Ruins is puzzled by those bricks because he wants to put those bricks into as many different addition equations form x+y=z as possible. Each brick can be used at most once and x, y, z are one digit integer. As Ruins is a beginer of addition operation, x, y and z will be single digit number. Two addition equations are different if any number