stackFun.js

/*

Given a river, determine whether it is crossable.

river = 'o~o~~o~~~o~~~~'
         S 2  3   4    5  => true

river = 'ooo~o'
         111 2
        'o~o~~o~~ooo~~'
         1 2  3  3  
o rock
~ water

At any point your speed is k
    Your next speed can be k - 1, k, or k + 1    

starting speed is 1

*/

function crossRiver(river) {
    let speed = 1;
    let moves = [];
    let visited = [];
    let i = 0;
    while (i < river.length) {
        if (i + speed + 1 >= river.length || i + speed >= river.length) return true;

        if (river[i + speed + 1] == 'o' && !visited[i + speed + 1]) {
            i = i + speed + 1;
            speed++;
            moves.push(speed + 1);
            visited[i] = true;
        } else if (river[i + speed] == 'o' && !visited[i + speed]) {
            i = i + speed;
            moves.push(speed);
            visited[i] = true;
        } else if (river[i + speed - 1] == 'o' && !visited[i + speed - 1]) {
            i = i + speed - 1;
            speed--;
            moves.push(speed - 1);
            visited[i] = true;
        } else if (moves.length) {
            i = i - moves.pop();
        } else {
            return false;
        }
    };
    return true;
}

console.log(crossRiver('o~o~~o~~oo~o~~~'));
console.log(crossRiver('o~o~~o~~~o~~~~'));
console.log(crossRiver('o~o~~o~~ooo~~'));