494. 目标和

494. 目标和

回溯backtrack，每个数字对他有两种操作

1. + nums[i]
2. - nums[i]

var findTargetSumWays = function (nums, target) {
    let res = 0;
    backtrack(0, 0);
    return res;
    function backtrack(i, sum) {
        if (i === nums.length) {
            if (sum === target) {
                res++;
            }
        } else {
            backtrack(i + 1, sum + nums[i]);
            backtrack(i + 1, sum - nums[i]);
        }
    }
};