第一步使用

// 说明
function isValid (str) {
    if (str.length % 2 !== 0) return false

    const stack = []

    // 遍历
    for (let i = 0; i < str.length; i++) {
        const s = str[i]

        if (s === '(' || s === '[' || s === '{') {
            stack.push(s)
        } else {
            const t = stack[stack.length - 1]
            if ((t === '(' && s === ')') || (t === '[' && s === ']') || (t === '{' && s === '}')) {
                   stack.pop() 
            } else {
                return false
            }
        }
    }

    return stack.length === 0
}